The Adventures of Jimmy Neutron, Boy Genius: When Jimmy shows up in a tux for Jet Fusion's wedding to Beautiful Gorgeous in "My Big Fat Spy Wedding", Cindy is, to put it simply, stunned by his beauty.
Cindy: So Neutron's the best man? Yeah, more like the best NERD! I bet he'll look like a—(She looks over and gasps. Jimmy shows up outside the church in a tux. Cue fade to pink background, heart-eyes from her, and hair patting to himself) ...HUNKMUFFIN.
She also says this to him when she arrives at his lab for a date and sees Jimmy dressed up nicely for the occasion.
Hayley from American Dad! has been shown to be pretty attractive when she wears a more flattering outfit and hairstyle, even developing a much curvier figure than her thin top and loose jeans would indicate. On few episodes, Roger even tells her that if she cleaned herself up and abandoned her hippy look, men would find her more attractive.
Amphibia: Anne Boonchuy usually goes around in her dirty school uniform and only one shoe, with leaves and twigs perpetually stuck in in her hair, but whenever she gets a new outfits and grooms herself for a special occasion, be it a girls' night out, a dance, or helping revitalize a restaurant, she looks positively stunning.
One episode has Ginger being forced to impersonate Courtney Grippling and so gets a makeover. When she walks into the kitchen with her hair done like Courtney and wearing stylish clothes, her mother says "wow, look at you". Although it's presented as a negative thing since Ginger starts acting more like Courtney and forgetting who she is.
An episode with a High-School Dance averted this for most of the girls, though Macie did make Dodie and Chett's mouths widen when she showed up in her dress. Ginger and Dodie also have this reaction to Macie when she arrives for their graduation.
Avatar: The Last Airbender did it in the episode "City of Walls and Secrets", with both Katara and Toph all dolled up and Aang stunned into blushing silence and a stumbling compliment before they had even left for the party.
A more subtle example would be Zuko at the start of season three. After spending the first season being kinda ugly and the second being dirty and scruffy, the viewers finally got to see him all dressed up and prince-like.◊
This also applies to Avatar Korra. While a tomboy normally, when she puts on make-up and a gown for a date or formal ball, she ranges from elegant to stunning.
In the Betty Boop cartoon, "Sally Swing", Betty transforms a rather plain looking cleaning woman into the stunning Sally Swing.
Ben 10: Alien Force: A both-genders instance happens for Gwen and Kevin at the end of one episode.
Zig-Zagged in Clone High when first Abe, then Toots, then finally Cleo give Joan a makeover. The former two have horrible taste (Abe makes her over as a sexy nurse with a cyborg eye cribbed from a Fangoria cover) and are blind respectively, and make her look terrible. Cleo, who has excellent fashion sense, nevertheless makes her over to look like a cheap hooker, but Abe reacts as though she looked elegant and refined. Kennedy claims to like her better in her usual tomboy / Perky Goth attire, but he may have been lying to get in her pants. In the same episode, JFK makes over Gandhi in his own preppy style, which is a hit with the ladies in-universe, although to the audience he is better-looking later on in a white tux with white sneakers, although in that getup he strikes out.
Danny Phantom: Tucker gets to experience the tongue-tied moment rather than Danny when Sam dolls up for a party.
Daria invokes this trope in order to convince Quinn to stop posing as an intellectual.
Family Guy: The episode "Don't Make Me Over" had Meg Griffin get a makeover that made her look pretty. She reverted to her normal self because it was too much work. Meanwhile, in the episode "Blind Ambition", Lois stops wearing makeup after Peter goes blind, revealing that without it she looks pale and wrinkled.
In Foster's Home for Imaginary Friends, Frankie was never lacking in the looks department, but when she dresses up for a date in "Frankie My Dear," Mac and Bloo begin fighting for her affection.
Laney usually sports a tough rebel-type appearance that makes her look somewhat boyish, but when she participates in a beauty pageant, and Kin and Kon work on her dress, hair, and makeup, she ends up looking absolutely beautiful in a black and green dress and her hair is done in two side buns. The usually Oblivious to Love Corey looks lovestruck when he sees her looking like a real girl for once.
"I gotta admit. You make a cute girl... Bro!"
Male example: Corey, like Laney, has a similarly rebel-styled look, but he looks very dandy when he dresses all fancy for a wedding gig, with a clean blue suit, a bow tie, and his trademark skull beanie gone to reveal a nice clean haircut. Laney also gets a bit fancier for the same gig, putting on a bow in her hair and a skirt that makes her look quite darling.
Hey Arnold!: This trope is parodied in the episode Helga's Makeover, in which the normally tomboyish Helga subjects herself to a girly makeover using tips from a teen magazine in order to join an all-girls' slumber party.
Also, in Arnold's Valentine, she tries to disguise herself as a French girl, which ends with her sporting a literal poodle hairdo that looks as ridiculous as it sounds. Ironically, when her hair gets messed up, she actually does end up looking quite pretty, and there are other instances, such as Helga's Boyfriend, in which she can pull off genuinely good looks.
In the episode where they play Romeo and Juliet, Arnold tells her she looks very nice in her costume.
Jimmy Two-Shoes: Heloise looks more like a little girl compared to other examples (Word of God is that she's about Jimmy's age though), but episodes like "Scent of a Heinous" and "Heloise's Secret Admirer" have shown that even the normally Oblivious to Love Jimmy notices how much more elegant she looks when dressed up for formal occasions.
KaBlam!: June, the tomboyish female host, dislikes wearing dresses unless she's made to wear one, but she does look pretty in a dress.
Kim Possible did this one, on two separate occasions, on the stairs at her house. There's no grand staircases in High School gymnasiums, so one would think that the real-life temptation to attempt this trope for a prom date is very hard to fight for a teenage girl.
The Magic School Bus: Wanda manages to pull this off any time she dusts off her pink tuxedo for a formal occasion.
My Life as a Teenage Robot: Jenny's not lacking in the looks department for a robot, but in "The Great Unwashed", she gets a real makeover in hot red from some mechanics after being wrecked in an explosion. Even Brad is stunned by the results.
There's also Rainbow Dash whenever she dresses up for big occasions, most notable include the Grand Galloping Gala and Shining Armor and Princess Cadence's wedding. Fancy Pants has commented on Ponyville being "charmingly rustic" which means he's well aware of how country folk like Applejack and Rainbow Dash can look gorgeous. Cadence herself looks pretty beat up and defeated when we first meet the real her but by the end, looks fabulous in her wedding gown.
Fluttershy also looks stunning whenever she dresses up, though she is already cute naturally. No wonder Rarity asked HER to be her model in "Green Isn't Your Color".
Candace in Phineas and Ferb. Several episodes, such as "Gaming the System," show that she is very lovely with a different outfit and hairstyle.
In The Powerpuff Girls episode "Octi Gone", the girls all wear party dress versions of the normal clothes. Bubbles and Buttercup are even decked out in hair bows. Of course, Buttercup is the one to complain:
Professor Utonium: I sure do love parties, don’t you, girls? Buttercup: No! Cuz I have to wear this stupid girly party dress and I look like a dork!
In Recess, the Ashleys enter rough-and-tumble tomboy Spinelli into a beauty pageant as a prank. After a makeover and pageantry lessons from her friends, Spinelli turns out to be quite good at the beauty pageant thing when she sets her mind to it and even wins the pageant.
In the Silly Symphonies short “The Cookie Carnival” we find a cookie girl who is rather homely looking and is dressed in rags, she’s sad that she can’t enter the beauty contest because she doesn’t have a fancy dress, a wandering hobo dolls her up and decorates her with various frosting, custard, sprinkles, and other assorted candy, she becomes very pretty, now looking like a young blonde woman, she’s so beautiful that she ends up winning the contest and everyone wants to marry her.
The Simpsons has Lisa, in "Lisa the Beauty Queen" convinced that she's ugly. In an attempt to rectify this, Homer enters her in a beauty pageant. Lisa competes, and wins second place. Bart more or less tells her in this episode that she is quite pretty.
Bart is also a bit overcome when he sees her in her wedding finery, in the future-based episode about marrying a boy she meets in college. He tells her that seeing her makes him want to get married for a third time.
Smiling Friends: Desmond spends most of his episode being an over-detailed and overweight Gonk whose poor hygiene and lack of self-care reflects utterly in his appearance... when he finds a purpose in life that pulls him out of his funk, however, he cleans up completely (even regrowing his hair) and starts looking a lot more respectable.
South Park: Subverted in "The List", when Kyle starts hanging out with the ugly kids, and tells them that they just need a new look. He tells a girl to stop wearing glasses, but that makes her more ugly (as her eyes look like tiny pinpricks instead of the standard design for the show's characters).
SpongeBob SquarePants makes himself over to be "long, tan and handsome"note "long" being the underwater substitute for "tall" to chaperone Mr. Krabs's daughter Pearl to her prom, by way of a wig and stilts. Mr. Krabs even comments, "Cleans up pretty good, doesn't he?" when he sees the new look.
While Star is typically cute, her getting gussied up for a fancy ball In "Blood Moon Ball" is the first time her friend Marco compliments her looks. Star's demon ex-boyfriend Tom nails the trope's name upon seeing her later in the same episode.
In "Bon Bon the Birthday Clown", Jackie and Star are left speechless when they see Marco dressed up for a high school dance. This is also when Star's subconscious crush on Marco starts becoming conscious.
Also in "Bon Bon the Birthday Clown", Jackie, Marco's date. While she is usually a tomboyish skater girl, in her dress she (in Star's words) "look[s] like the ocean!"
This happens to the main trio when they prepare to go to their high school's prom in Sym-Bionic Titan: Not as much as Ilana, who usually is dressed very nicely, but Lance's hair is groomed and Octus (while trying on outfits) is able to alter his appearance to look like he's in shape.
Teenage Mutant Ninja Turtles (1987): Occurs to April after Irma insists she dress up for a royal ball at an embassy. All four turtles go gaga over her new look and despite the fact that she's never been a slouch in the looks department, and the fact that they're... well, turtles.
In the 2003 series Casey Jones looks damn sexy in a tux.
Robin experiences this with Starfire, when she unexpectedly shows up at the party all dolled up so she can "investigate Kitten (the episode's antagonist) thoroughly". Whatever that means.
In the same episode, Robin himself looks quite nice in that tux.
There's another instance for Starfire, in the episode "Go!". Throughout the entire episode, Starfire has been wearing her prison garb and either screaming at everyone or being suspicious of them (since she just escaped being sold into slavery, that's understandable). At the end, when the enemy has been driven away, Starfire shows up wearing the outfit she spends most of the series wearing, smiling and looking happy and relaxed for the first time. Everyone is amazed.
Played with in episode 4 of Wakfu. When Evangelyne lets down her hair and puts on a princess gown, she thinks she must look ridiculous, but her friends are impressed. There are a few problems with this, though: they are supposed to disguise as "ugly" princesses, and Eva's cuteness makes her suspicious. Also, she can barely walks in high heels and falls several times, making the stairs part of the trope quite perilous. As for the Love Interest, Sadlygrove, he sure finds her good-looking... although he doesn't even recognize her.
Done in an episode of The Weekenders where Lor develops a crush on a boy and starts acting more girly to impress him including wearing dresses and makeup. Even Tino is shocked at how good she looks.
Shriek from CatDog, when she goes to CatDog's cookout in a dress and her hair made up in an attempt to impress Dog. It results in nearly every guy (except for Dog) trying to hit on her, and when they realized who she was, they all fainted. As it turns out, prior to this episode, Dog didn't even know that Shriek was a girl.
We Bare Bears: In "Bubble" Charlie the sasquatch, after getting hosed down with water after saving Panda from a polluted lake, is revealed to have shiny golden fur. He prefers his dirty, unkempt fur because being so clean makes him uncomfortable.
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