The DCU's Ambush Bug qualifies as well. He first appeared as a fairly standard supervillain, but within just a couple appearances had jumped into outright comedy, making fun of the fan obsessions of the day.
It's rumored that Dan DiDio, DC's current Editor-in-Chief, dislikes "silly" comics, which is why many ex-JLI members have died under his reign (well, the lucky ones died, anyway); it's more likely due to the fact that most of them haven't been in the public eye for years, leading them to becoming C-List Fodder.
Meanwhile, Gold and Rip Hunter are actively exploiting Booster's reputation as a second-rate hero to let him fight time-traveling opponents who would smother Booster in his cradle if only they figured out he was the problem.
Keith Giffen also fathered Lobo with Roger Slifer, which is a Badass Biker coupled with professional wrestling camp and biker humor. The plot is always silly, including Lobo working as a mascot in a crappy amusement park, or the time the writer is wired to a jokes detector shocking him for each "adult joke", turning Lobo into Superbo, a Silver Age Superman ripoff to make him family-friendly, or other nonsensical stuff as the crossover Lobo vs The Mask, or the mini-series Lobo's Back, about his failed resurrections.
Stephanie Brown aka Spoiler / Robin / Batgirl is generally regarded as this amongst the Batfamily, beating out Dick Grayson as the most optimistic, least angst-ridden member.
A subversion of this kind of character comes in the form of Kid Devil. Joining DC's Teen Titans after Kid Flash left, he was made to seem like the comic relief member of the team. However, unlike most of these characters, he was neurotic, envious, and very self-conscious. He projected a fun, optimistic attitude to hide his insecurities and the fact that he had sold his soul to get his powers. These shortcomings did a lot to make him more than just Kid Flash's replacement. His teammate Miss Martian comes closer to the spirit of the trope.
Miss Martian was from the evil white martian race, but rejected that entirely to be one of the most bubbly characters in comics. At one point she got an evil side in her head, who she fought off via happy thoughts of puppies.
Wonder WomanVol 1: Even when talking about his past of being enslaved and helping lead a slave revolt "Glitch" manages to be upbeat. This attitude does not wane when his people's former enslavers show back up and he joins the fight against them, only sounding a bit stressed when he thinks he's put himself in a spot where he has to die in order to save his new human friends.