Ironically, he eventually becomes The Strategist for the Master Builders. And not only that, the double-decker couch saves their lives because it was so stupid, the Big Bad missed it. Additionally, the couch is the only object that survives the destruction of the submarine built by the Master Builders. Why? Because, while they're great a building things individually, working together doesn't come naturally to them, and their individual styles conflict with one another.
Also, when Wildstyle is explaining Lord Business' plan, he hears something else, and then he says:
Emmet: I think I got it, but just in case, tell me the whole thing again. I wasn't listening.
In one sense, The Nightmare Before Christmas turns this trope on its head because one of the main causes of all the trouble the characters get into is not Jack's stupidity, but his being smarter than everyone else in Halloweentown. Jack is much too worldly and sophisticated to limit his cultural horizons to his provincial hometown, and his desire for cultural diversity is what motivates him to try to combine Halloween and Christmas. In this light, it's the supposedly naive Sally who comes off as the smartest one.
Santa Claus: The next time you get the urge to take over someone else's holiday, I'd listen to her! She's the only one who makes sense around this insane asylum!
Reggie from Free Birds is a dimwittedegoist who thinks he's better than anyone else. C'mon, the guy doesn't even know what TV is.
Forrest Gump. He actually appears to succeed becauseof his idiocy, or rather, because it never seems to occur to him to do anything other than the Right Thing by traditional standards. He becomes a decorated war hero just by trying to save his Army buddy Bubba and his comrades. His shrimping business succeeds only because a hurricane has sunk every boat in the harbor but his; it is strongly implied this is because Gump has been going to church lately and got God on his side. Other characters who do not share his conventional morality, and explore alternative values and lifestyles as so many Americans did in the 1960s and '70s, act like total bastards and/or make fools of themselves. Even the good-hearted Jenny succumbs to Death by Sex, since she gets AIDS.
Wikus van de Merwe from District 9. Let's just say that none of his boneheaded decisions throughout the film are out of character. Though it's more a case of Wikus being a Dirty Coward at best and Villain Protagonist at worst.
Jake Sully from Avatar. His main qualification for the Avatar program was being his scientist brother's twin. His military training and brash personality cause major clashes with other characters.
Star Trek (2009) Kirk seems to be playing with this one. Unlike TOS Kirk, the reboot Kirk needs his supporting cast to keep from getting his ass handed to him every five minutes (Spock, Spock Prime, Sulu, Scotty, Bones, and Chekov keep Kirk from killing himself in various interesting ways while Hypercompetent Sidekick Uhura holds the ship together!). Likely justified, due to being raised without a father.
Lt. Frank Drebin from The Naked Gun is fairly competent, but given he lives in a slapstick world, he's always prone to dumb sayings, decisions, pratfalls (though once it helped, as Frank tripped on the power cord of a bomb and stopped it), and humiliating things.
Adam Sandler in most of his movie roles. Billy Madison even has the justification of having dropped out of grad school. Pixels has Sandler excel due to knowing old-school video games.
Jack from Jack the Giant Slayer is a wide-eyed and gullible farmboy who nevertheless grows to be a true hero.
Vincent Vega from Pulp Fiction is this at best; his idiocy gets himself and a witness to one of his hits killed and also comes dangerously close to getting Mia Wallace killed.
Bill & Ted, in every incarnation. While they have an intuitive understanding of San Dimas Time, their basic response to everything is almost always (a) tell a dumb joke, (b) make a rock music reference, (c) going along with what someone smarter suggests, or (d) some combination of the above.
Patchi from Walking with Dinosaurs: The Movie was this at first, a chronic complainer with poor survival skills. He finally shapes up when he leads the herd off the frozen lake, the same lake Scowler is leading everyone across.
Valentina: He'll try to confuse you, mess with your mind. Derek: Don't worry. That is... closed for business.
Clumsy, slow-witted Bumbling Dad Gang-du from The Host is a tragicomic example. He's constantly viewed as the family screw-up, and because he's such an idiot he endangers lives without meaning to. He even got his father killed after he miscounted the number of bullets in his gun. Still, he can be a Crouching Moron, Hidden Badass when the situation calls for it.
Ship of Monsters has Lauriano, who doesn't even realize what's going on until the last minute. He wins mostly through luck.
None of the protagonists in the two Dr. Goldfoot movies are particularly bright, but Franco and Ciccio in Dr. Goldfoot and the Girl Bombs are an Italian comedy duo that make Laurel and Hardy look like intellectuals.