In Astro City, Crackerjack subverts this trope. He is a skilled brawler, excellent acrobat and master of disguise, as well as a true hero in every sense, but hearing him talk, the resident Superman analogue is a poseur next to him. An alien assumes his arrogance and bragging indicate his true character, and even when seeing his heroism wrestles with the idea that he might really be The Hero.
Volstagg the Voluminous, of Marvel Comics' Warriors Three. A blowhard who called himself "The Lion of Asgard" and lays claim to several improbable feats of strength, cunning, and bravery, he proves ineffectual and cowardly in actual combat. In his own defense, he often mentioned that these supposed exploits took place when he was younger and more physically formidable. More recently in the comics, he has been retooled into a Boisterous Bruiser, fond of food, drink, and women and given to much merriment, in more equal contrast with his friends Fandral the Dashing and Hogun the Grim. He's still a braggart—such as considering himself the equal of any three or four human Avengers—but not for no reason.
He also has his moments of badassery, typically when his back is against the wall, when his friends are in danger, or simply due to his own clumsyness. He has also been mind controlled into fighting Thor before and proven himself a good challenge to the God of Thunder.
Both averted and subverted in the movie: although he still has a hefty appetite, Volstagg is able to carry himself in a fight as well as any of Thor's companions (it takes a veritable dog pile of Asgardian soldiers to subdue him when covering Thor's escape with Loki), averting the trope (but subverting the character as written in some of the comics).
Several of the members of the new Infinity, Inc. in 52 were very fearless when they were Lex Luthor's media darlings beloved by the public and taking down minor thugs to widespread acclaim, even going so far as to smugly lecture members of the Justice Society that they were the new generation replacing them and that there was nothing they could do about it. Then, when Black Adam declared war on the world and a real crisis ensued, they were last seen cowering in some rubble before running away.
The dashing and charming warrior Arcadio from Sergio Aragones' Groo the Wanderer. Despite the quests that Arcadio embarks on, he is almost never the one to actually complete them. Yet will both take the credit and believe that he completed them on his own regardless.
The Rival Herminus in Swordquest frequently boasts about his expert thieving skills and cutthroat nature, but never demonstrates anything out of the ordinary.