Spider-Man is one of Marvel's light-hearted and prominent cheerful Nice Guys, and the most upbeat of The Avengers. Despite his angsty and troubled life, Peter Parker most of the time takes serious and depressing situations and turns them into amusing cracks at people and humorous antics at any given time.
So is his Love Interest Mary Jane Watson, before it was later revealed that this was all to cover up her insecurities and troubled life at home.
Although She-Hulk is more prominent, Marvel's flagship character for this trope is Doreen Green, better known as Squirrel Girl, whose silly powers and low actual presence in comics nonetheless have helped her beat a number of supervillains. She actually defeated Dr. Doom once, which lead one writer to retcon that as being Actually a Doombot which led another writer to have her defeat Thanos offscreen and then have Uatu the Watcher show up to officially proclaim it as "not a robot, clone, or simulacrum."
Just to show how the feud continues, subsequent comics have had Thanos reveal that he has perfected a means of creating weaker copies of himself that could fool The Watchers.
Both She-Hulk's current comic and all of Squirrel Girl's recent appearances are written by Dan Slott, who pretty much epitomizes the "comics should be fun!" attitude. Slott wrote both the above mentioned items, thus feuding with himself!
Squirrel Girl is infatuated with Robert Baldwin, the superhero known as Speedball, who was also one of these for most of his career — prior to turning Darker and Edgier as Penance. He's Speedball again now, but he's still not quite his old fun-loving self.
Marvel's Deadpool started out as Rob Liefeld's stand-in for DC's Deathstroke the Terminator. Nearly every other writer since has used him as a comedy character, particularly in the areas of Medium Awareness and various attacks on the fourth wall. His Ultimate Marvel counterpart is (almost) entirely serious, but there are hints that might not have actually been him.
Irving Forbush (also known as his "superhero" persona, Forbush-Man) is an earlier Marvel character (circa 1967) who fits this trope. He was the main character of Not Brand Echh, a '60s superhero parody comic, and What The—!?, an '80s/'90s superhero parody comic.
Which makes the evil psychic clone of him in Nextwave all the funnier.
The Awesome Slapstick: A Marvel series where the title character is called a "living cartoon", and carries a Hyperspace Mallet. In one issue of Avengers: The Initiative, he was shown brutally attacking Camp Hammond instructor, Gauntlet, for using the New Warriors name as an insult, nearly killing him in the process. This has been seen by some as an Out-of-Character Moment, though others see it as not really being far off from his normal cartoon-ish prankster nature and shows his loyalty to his teammates, who he even tried admitting the act to, before getting interrupted. For now it seems that Iron Man villain Ghost has been blamed for the act.
Iceman, in an early issue of X-Men: First Class, calls himself "The Bringer of Fun", and throughout the series generally acts like a lovable dork.
In New X-Men this role is taken by both Santo (Rockslide) and Megan (Pixie). Megan who is just too cute and causes hallucinations of teddy bears and unicorns... to Wolverine. And Santo is the kind of jerk you gotta laugh at, roasting marshmallows over his classmate's head, or scaring his classmate worried about being killed by EXPLODING next to him on the sofa randomly.
Marvel has yet another fun personified character in the from of Morph from the Exiles.
Nightcrawler of the X-Men was originally goofier, liked having fun, and played pranks all the time. Some later versions of him downplay this quality or remove it entirely, depending on who is writing it.
The original Marvel Excalibur was an example of this trope until being reimagined as just another Dark Ages X-book. The original idea was the most "fun" members of the X-Men (Nightcrawler and Kitty Pryde) form their own team of European superheroes when it appears the other X-Men are killed. They were joined by other Fun Personified characters in the process.
Warlock of the New Mutants was (and since his recent resurrection, probably will be again) basically a walking scribble made of semi-organic circuitry, prone to bizarre behavior and random shapeshifting (the "can turn into anything" brand of shapeshifters are heavily represented in this trope).
