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The Incredible Hulk is usually portrayed as childlike in his simplicity. His original characterization, circa joining The Avengers, had him as more of a temperamental, rebellious teenager. Though note there is a difference between childlike and stupid, and the Savage Hulk can be remarkably cunning and shrewd when the situation calls for it.
Although, really, most Hulk villains besides the Leader and MODOK are brawny, brainless monstrosities.
This side of him is played up considerably in Twisted Toyfare Theater, which also features Bizarrobama, the Prime Minister of Canada and an imperfect clone of Barack Obama.
Captain Marvel's na´vetÚ tends to come across this way, but since he's a kid in an adult's body, it's excusable.
Captain Everything in normalman. Cap is, well, a moron who has been known to forget how to fly in midflight, but he's so childlike it'hard not to like him anyway. So, while he causes needless destruction and fights his own friends just because he thinks the scene needs action, he's still completely guileless and doesn't have a mean bone in his body.
Several Marvel Comics villains like the Rhino, the Sandman, the Blob, and the Ox.
The Ox can trounce She-Hulk with minimal effort and punch colossal holes in brick walls but doesn't grasp the concept of figurative language; when his and Boomerang's lawsuit against the Vision is settled "for an arm and a leg", he violently insists he wants cash and couldn't care less about having a robot's limbs.
Most incarnations of Killer Croc in Batman, particularly in Batman: The Animated Series and current comics. The exception here is The Batman, which, in addition to a Race Lift making him Cajun, made him a bona fide criminal mastermind of his own. This was actually truer to the original version of Croc introduced in the comics of the early '80s, before Flanderization set in and writers portrayed him as nothing but a savage brute. Though, on the other hand, Killer Croc in Batman: The Animated Series did pull off a very intelligent frame job against Harvey Bullock in his first appearance. Then he basically just became comic relief Dumb Muscle.
Other villains in the DCAU, like Rampage, Solomon Grundy, and (the first) Blockbuster, also count, as do their comics counterparts (though Rampage and Grundy are occasionally good guys).
There's also Ra's al Ghul's perennial henchman, Ubu. His purpose in life is simply to hit whatever Ra's tells him to.
And another Batman villain, Monsoon, who is very nearly but not quite retarded and responds to most prompts with a slurred "RIGHT RIGHT!"
As well as Amygdala, who is missing the part of the brain he's named for.
And their fellow Blackgate inmate Titus Czonka, the Baffler. Very stupid but phenomenally powerful, he made the mistake of trying to be a more traditional Batman villain as The Baffler, a second-rate ripoff of his cohort the Cluemaster who was himself a second-rate ripoff of the Riddler. So the stupid guy starts leaving ridiculously simple clues for the authorities to catch him with...
The Superman villain, Doomsday, barely has any mind to speak of at all. Due to the multiple deaths he suffered during the experiments that created him, Doomsday is little more than a frightened and enraged baby. Imagine an infant trying to swat away something scary with his little baby fist. Now imagine that said baby could topple a mountain with that swat and you've got Doomsday. The one time he was given intelligence it rendered him weaker because it exposed his true nature as an infantile coward terrified of death.
Though you wouldn't know it most of the time, this role is often filled by Wolverine of all people in the X-Men. Yes, the expert survivalist who speaks 17 languages. Of course, Wolverine's a moron in a more social sense, and while he is very knowledgeable in areas concerning, to be frank, killing guys, he can have trouble with almost anything else.
And several other X-Men, as well. Sunder of the Morlocks (virtually retarded, mountainous size and strength), Basilisk (in Xorn's "special" class, giant-sized, weight off the charts), Rockslide (a muted example; not the strongest X-Man around, but just a tad above "moronic" intelligence wise), and Onyxx (who seems to have stopped developing mentally around age 12).
Dumb Bunny, aka Mrs. Ambush Bug, from the Inferior Five. The name says it all.
She meets her match in the first issue of their own series: Mr. Mental is an extremely powerful telepath who is really really really stupid. "Try your helmet! Hit 'em with a blast of telekinetic power!" "Oh, is that what this is for? I thought we'd been playing soldier!"
Prester John (yes, that one) in Cable & Deadpool, a gigantically muscled man who dwarfs even Cable in size. Among other things, he is utterly perplexed at the motive behind the murder of the Marvel Comics equivalent of Osama bin Laden. As it was explained to him, "He wanted him to stop being the world's most wanted terrorist." His reply? "Well, it worked."
Dengar from Star Wars. A big, beefy lunkhead, he at least has the excuse of having suffered brain damage in a swoop bike race with Han Solo. Combine that with the loss of sanity from the experiments the Empire did on him afterward to program him as an assassin, it's actually a wonder that Dengar functions as well as he does.
The Flash villain Big Sir, who was born with severe mental deficiencies and a glandular problem that caused him to grow into an extremely large man of over eight feet. The Monitor gave him equipment to become a supervillain, taking care to include a headset that periodically emits a beeping noise so he'll remember to stay on task.
Block in Hero Camp, who is almost completely illiterate; When we're shown a book from his perspective, it's almost entirely gibberish and he can only understand a single sentence on the page. He's also so slow that he thinks his incredible weight is due to obesity, when he's actually made of solid rock without an ounce of fat on him.
Maul of the WildC.A.T.s actually gets dumber as he gets bigger. He once got so big he forgot how to return to normal size.
Maul is a bizarre case: while his power is that he can swap brains for brawn, in his "normal" form he's a nobel laureate: so, when he doesn't grow too much, he retains enough smarts to be more like a Genius Bruiser.
Gorgilla and Elektro of the Fin Fang Four; Elektro is a robot with a 32k processor (later upgraded to 64k; as Elsa Bloodstone puts it, "I've got JPEGs smarter than him."), while Gorgilla has grade-A Hulk Speak and intelligence. Both are also incredibly powerful and destructive given the proper motivation.
Mongrol and Mek-Quake from the ABC Warriors; Mongrol is a battered, punch drunk colossus whose mind was scarred by war and torture until he considered his brain to be a second party that he must occasionally parlay with, while Mek-Quake is a one mek wrecking crew whose only aptitude and only joy is in violent acts, known to him as "big jobs".
The Lifter (an airheaded superhuman weightlifter) and Mover (a Gentle Giant who lets his girlfriend Shaker do that thinking) from Wildguard. American Icon and the Romancer are arguable examples as well, while the Human Shield is a giant beast who only recently overcame his lifelong struggle with illiteracy.
Jitterjack from Astro City. To wit: his first words on panel are "HURR HURR". The combined version of two men whose minds were turned to mush as they were unified, but with strength far beyond that of any two ordinary men.
Lead of DC's Metal Men is a very strong example, being incapable of completing a thought without stumbling through it with several "Uhmm"s as well as being the toughest of the group (though Iron is slightly more powerful).
The mysterious Zombie, alias John Doe, from Nick Fury's Howling Commandos. Big, strong, and with extremely stunted intelligence. This might actually be a Stealth Pun: Zombie could use some brains.
Played with in Sin City. Marv is not educated and considers himself stupid but he displays a knack for solving mysteries and figuring out a way to defeat a skilled opponent.
Yeagar from Nodwick is the party fighter and has sub-par intelligence and wisdom scores. In-story he tends to get confused by big words, metaphysical concepts, or most things that cannot be treated by stabbing, imbibing, romancing or looting it.
Thoroughly subverted in The Ravages Of Time, it's Genius Bruiser which is closer to the norm. After all, the "big dumb brutes" from the novel? What fool would leave them in command of an army? Lu Bu and Zhang Fei are the most prominent examples of this subversion, but as a general rule, if someone looks like Dumb Muscle they're almost certainly faking it... yes, even Xu Chu.
DCU villain General Wade Eiling was originally a very clever villain but was reduced to this when he transferred his brain patterns to a Shaggy Man body to avoid dying of cancer. The Shaggy Man's brain was incompatible with Eiling's mind and he became a dumb brute as a result.