Aquaman: Aquaman and Mera are almost always shown in costume, even when just hanging out at home. This makes a certain degree of sense since they don’t have secret identities, so might as well always be dressed for a fight. It doesn’t look out of place when they’re in Atlantis, but it’s mildly surreal to see them eating in a restaurant or grocery shopping on the surface complete with tiara and trident.
Asterix: Asterix gets a different hairstyle for a while in The Actress, Cacofonix gets a scarf all through The Roman Agent to indicate that he's lost his voice and occasionally Geriatrix's wife will try out a new fashion trend or Julius Caesar will be shown wearing some new piece of ceremonial dress, but for the most part this is completely followed. Taken to the point of ridiculousness in Obelix All At Sea, when Asterix asks the village seamstress for a new set of clothes for the magically de-aged Obelix, and she makes him kid-sized versions of his usual outfit, explaining that the child looks so much like Obelix that she couldn't resist making him striped breeches too.
In the earlier books, characters other than Asterix, Obelix, and Getafix had more variation in outfits, although this was changed in revised editions.
The Beano: Pretty much every character wears the same clothes all the time; however, their clothes have occasionally changed. For example, Dennis the Menace originally had a little tie back in 1951, but his clothes changed to a stripy jumper and then to a black and red jumper once his strip gained the colour red, and his clothes have never changed since except for the occasional gag about how old fashioned his clothes are. This also true for a number of strips, especially The Bash Street Kids, where Teacher has worn an old fashioned Teacher's outfit complete with mortarboard since the 1950s, which is frequently lampshaded.
Calvin and Hobbes: Calvin has his trademark ensemble of red striped t-shirt, black short pants, and white-and-red sneakers. As it happens, he does have a dresser full of other clothes—unless said clothes are strewn about his room—but is never seen choosing to wear them. He also has pajamas and winter wear. All other characters do change their clothes frequently.
Dilbert: Everyone wears the same clothes every day.
In some cases, even for casual day:
Wally: Well, well. It wouldn't be casual day without Alice wearing her one pair of tan pants.
There are a couple of strips that imply it's because they have zero fashion sense, so by trial and error they've discovered a combination that doesn't get laughed at too much and bought dozens of copies of it. In one strip mocking the concept of "casual day", Dilbert and Wally show up in bell bottoms (due to cyclic fashion trends, it should be explained that this was at a time when bell bottoms were very definitely "out").
Doonesbury: B.D. always wore his football helmet 24/7. Then when he was in the First Gulf War he switched to a "Fritz" helmet. He wore the helmet until he was wounded and discharged from the army. Lampshaded when they had to have a special operation to remove his helmet. B.D.'s helmet was a slowly changing icon. He switched to an army helmet for the first time when he went to Vietnam. Since then, he's also sported an NFL helmet, a CHP helmet, and a riotgear helmet (following the Rodney King verdict), among others.
FoxTrot: The series oddly plays this straight with some characters, but averts it with others. Peter Fox almost always wears jeans, the same sweatshirt and a baseball cap with the letter "A" on it. Likewise, Eileen Jacobson always seems to wear the same shirt and skirt.
Lampshaded in a strip where Jason got hand-me-downs from Peter. In the final frame, Jason is wearing a miniature version of Peter's regular outfit.
Jason: It's high time someone asked you a question... Peter: Lookin' good!
Paige's friend Nicole often appears in a white shirt with a black vest, although this is averted about as frequently as it's played straight. In the comic's early days in the '80s, Paige usually appeared wearing pearls.
Iznogoud: Unless he makes a point of wearing a disguise, Iznogoud is always depicted in the same outfit.
Luann: Averted with most characters, save for Brad (who seems to have multiple copies of the same black jersey) and Mr. Gray, who in all his appearances has only ever worn the same drab gray suit/black necktie combo. Even for his own wedding, all he did was add a boutonnière.
Lucky Luke: Lucky Luke is almost always dressed the same. Lampshaded on one occasion when he does find himself in an all-white outfit - and immediately goes into a store, demanding exactly the items he usually wears.
Peanuts: Pretty much all the characters. There's a discussion on the Depending on the Artist page where someone listed Charlie Brown's shirt sometimes being red instead of yellow; someone brought up the possibility that good old Chuck might actually have two zigzag-striped shirts of different colors. One of the cartoons did show Charlie Brown's closet, in which he had shirts in a variety of colors, all of which have the same black zigzag stripe on them.
In a December 1952 strip, Violet comments on how he wears the same shirt every day, so he comes back with a black shirt with a yellow zigzag.
Red Hood and the Outlaws: Surprisingly averted even in-costume by Arsenal, who has a lot of distinct baseball caps. So far, no civilian outfit has been used more than once by any of the cast, barring Red Hood's All Robes.
Ric Hochet: Everyone wears the same clothes, except for Nadine. Ric Hochet still has the same blue jeans, red shirt and white jacket from 1955.
The Sandman: Morpheus can wear pretty much anything he or whoever is looking at him can imagine, but when he's alone, he always wears the same emo ensemble of black T-shirt, black pants, black trench coat, and black boots (not unlike his author, Neil Gaiman). His sister is much the same, with a tank-top instead of a T-shirt and an ankh necklace.
Silver Surfer: Dawn Greenwood is a partial example. Her default outfit is a red minidress with black polka dots and collar, a black belt and leggings and red sneakers. She sometimes wears different outfits, but they're still red with black polka-dots. Her twin sister Eve favours yellow outfits with black stripes. It's eventually explained that when they were babies, their Dad got ladybug and bumblebee onesies to distinguish them, and that became their "thing".
The Simpsons: In issue #85, a pair of marketing experts decide that the Simpsons are the perfect indicator of popular culture, and model all of their company's products - including clothes - after their lifestyle. Cut to everyone in Springfield dressed like the Simpsons. After everything returns to normal, Marge reveals that she bought all of the duplicates of their normal wear. In the final panel, a small box with a Matt Groening pops up and Groening says, "And that's why the Simpsons always wear the same clothes!"
While most supervillains have actual costumes and can thus be a little justified in this, Ox of the Enforcers just wears a distinctive set of normal clothes that serves the same function but without the justification. Apparently he just likes turtlenecks, vests, and khaki slacks. The other Enforcers have a similar deal going on but tend to mix it up a little more often.
The Sandman is iconically associated with his brown slacks and green horizontally striped shirt; an early attempt to give him a more traditional supervillain costume never caught on. In this case, it really is justified - that's part of his body after the accident that gave him his powers.
Star Wars (Marvel 1977): For the first few years, the main characters generally just wore what they wore in the first movie. It wasn't until The Empire Strikes Back came out that the artists really began drawing the heroes in new outfits.
Steelgrip Starkey and the All-Purpose Power Tool: Steelgrip Starkey and his partner Flynn "Flyin'" Ryan almost always wear the same clothes — a red short-sleeve shirt and blue jeans for Steelgrip, and overalls and a white T-shirt for Flynn. In Steelgrip's case it's because it's his company uniform.
The Tick: Sometimes the Tick wears clothes over his blue (outfit? body?), such as a tuxedo at Dot and Neil's wedding, but he never takes it off. Possibly a Clingy Costume.
He seems to think it is. He's said at least once that it's not a costume, and he is "simply The Tick."
Wonder WomanVol 1: Even when the other Holliday Girls change outfits Etta Candy wears her Holliday College tank top and red shorts. Even in space. The only exception to the rule is if she's on vacation at her family ranch in Texas, in which case she wears her cowgirl outfit, or in the two issues which are costumers (and in one of those issues she wore her cowgirl outfit).
X-Wing Rogue Squadron: Wedge Antilles basically wears only three outfits: orange Republic flightsuit when flying, white-and-black formal uniform in formal occasions, and his civilian clothes, which consist of black boots, blue pants, brown jacket, and a vertically striped turtleneck shirt. Anything else, and he needs a plot reason to wear it.
In classic Superman comics, Clark Kent always wears a blue suit, white shirt, and red tie. This was actually explained once: When he changes to Superman, Clark compresses his street clothes into a flat packet which he carries in a pouch in his cape. To prevent them becoming wrinkled, he has to treat them with a special chemical which has the side effect of dyeing everything red, white, or blue...
Peanuts characters occasionally comment on their own unvarying clothing. How many yellow zigzag shirts does Charlie Brown own? And how would we know Rerun from Linus without the overalls?
The Peanuts Movie has two scenes in which Charlie Brown looks through a closet filled with yellow zigzag shirts, trying to decide on the right yellow zigzag shirt.
The closet gag was explicitly used by Jughead Jones in the Archie comic book series, where his closet has all clothes, which are all the same.
Katy Keene usually averts this trope; the main appeal of her comics is that her outfits are designed and sent in by readers, so she changes outfits very frequently.
Pete Wisdom in Excalibur once explained to Kitty Pryde that his entire closet contains nothing but black slacks, black suit jackets, and white shirts, because he wrecks so many clothes on missions that if he didn't do that he'd have a garish nightmare of partial sets of clothes. He ended up giving one of his outfits to Douglock when Doug had no clothes of his own to wear, causing Douglock to note that Pete said he had "twenty-seven suits of clothing that are all exactly the same."
Dykes to Watch Out For: Only really afflicts the main character Mo, who constantly wears jeans and a black-and-white striped top. Repeatedly lampshaded ("just wear another striped T-shirt and indestructible jeans," "you could get management to pay you enough to buy another outfit," "here's a fetching striped one"). In one episode, she dresses up to look nice for another character she has a crush on: this involves wearing a black shirt with thinner white stripes.
How much this trope applies varies from character to character in Luann, but Gunther has admitted to owning seven identical shirts.
Pointed out by Wally West in one of the comics based on the Young Justice (2010) cartoon when shopping with Superboy. "But don't you want a little variety? I mean you picked out a whole rack of the same black T-shirt..."
Zits takes this up to eleven by showing Jeremy and Connie shopping at a strip mall with a dedicated store for each item in Jeremy's Limited Wardrobe. Connie once claimed that men don't shop, they Tivo their closets.
The Shazam! comics have Billy Batson, one of the few mainstream superheroes who embodies this trope in his secret identity. He has a signature outfit in every time period and continuity, and in just about any of them, it'll be blue jeans with a shirt/sweater/jacket that's mostly red, possibly with yellow trimming and/or a white shirt under it.
Lampshaded in both Power of Shazam and Superman/Shazam: First Thunder, saying that when he was homeless he got a bargain for a dozen identical shirts.
Scott Here's your laundry, man...your eleven identical red shirts, all clean.
Billy: Gimme a break! It was twelve shirts for a dollar. Who cares if I wear the same thing everyday? It worked for Albert Einstein.
In the mainstream universe, Billy eventually learns that he can change Captain Marvel's clothes (and hair and beard) by concentrating when he transforms. So the superhero identity has an Unlimited Wardrobe, but the normal kid doesn't.
The Wacky Adventures of Pedro's title character usually only wears purple sweaters, and sometimes explains in the Boys' Life fan mail column that he owns more than one.
In Ernie Bushmiller's Nancy comic strip, the title character almost invariably wore the same outfit of white blouse, black pullover vest and red skirt. There was an attempt to hang a lampshade on that in one strip, in which Nancy is examining her closet full of identical outfits, but one newspaper that did its own coloring in its daily comics section knocked the lampshade off when a clueless colorist gave each outfit a different color scheme.
Exceptions
Whilst everyone else in Calvin and Hobbes changed clothes regularly (well, Calvin's dad did favor collared button-downs and black slacks), Calvin always wore the same red stripy T-shirt and black jeans, unless the plot required a change (e.g. bedtime, snow, etc.).
Lampshaded by Hobbes, when at one point he asks Calvin why he doesn't wear shorts in the summer. Calvin yells back that short pants touch his feet, but that doesn't explain why they're always the same pants.
Calvin wasn't the only exception, actually - there was also his bully of the strip, Moe. He always wore the same black T-shirt and pants every single time he appeared - except in one gym Sunday strip, in which he is seen in a white T-shirt with red shorts.
The original Wasp, Janet van Dyne, was an important exception. She changed her costume all the time. Eventually, anyway. When first introduced, she had a uniform that was always the same, but it began to evolve over the years. First she lost the pointy wimple, revealing her hair while everything remained as it was; then her costume changed occasionally, but with long periods between alterations; and then she went nuts and her costume became just one more outfit subject to the whims of fashion.
Phoebe in Phoebe and Her Unicorn have different outfits. Also, sometimes she ties her hair in a pigtail, while in other days she does not. The latter is lampshaded in this strip.
The creators of W.I.T.C.H. made not only sure to give their protagonists signature styles for their outfits, but even gave Hay Lin the habit (and skills) of making her own clothes once in a while, and the few who do play the trope straight have their own good reasons.
The Runaways wear different outfits in almost every story. The only explanation given for their seemingly endless wardrobes is that Nico designs and makes clothing as a hobby.
For a period in the 1960s, Supergirl would change outfits every few issues. All of the costumes were designed by fans. Eventually they decided on one costume and she kept that outfit (with minor changes) until her death in the 1980s.
De Rechter: With only a handful of exceptions, the characters are always wearing their togas, even when they are not at work.
Probably one of the earliest uses of the closet gag.
