Virtually every character played by famed comedian Lou Costello of the Abbott and Costello comedy duo. Occasionally he will even lampshade himself, such as when one of his characters in the film Who Done It? turns on a radio and hears "Who's on First?" (one of Abbott and Costello's most famous routines) and immediately turns it off, remarking how stupid the "short, chubby guy" (actually Costello himself) is.
Though he turns out to be a bit of a Genius Ditz in Wake Up Ron Burgundy (a bonus 'film' on made of deleted Anchorman scenes)
In Back to the Future Part II, we learn that Marty's son Junior is a total moron. Probably it has to do with the fact Marty had become a "chicken" after his car accident in the coinciding 1985 timeline.
Bullshot: "I see. You intend taking on the Most Dangerous Man in Europe by yourself, do you? Have you given a moment's thought as to what you intend using for brains?"
Rosemary: "How dare you! I've done pretty well without brains so far!"
August in Cemetery Gates. When Kym tells Tony and Enrique that they wouldn't now what to do with a pragmatic woman, August comments that she is part Russian. Later, Tony persuades her to make a sex tape with him with the argument "everyone is doing it".
Cher Horowitz in Clueless... kind of. She's certainly no genius level intellect, and is definitely more than a little naive and 'clueless', but she's savvy enough when she needs to be and has enough wits about her to 'negotiate' her grades with most of her teachers.
Corky Romano: Corky is a total goofball who's very nice but also extremely quirky and clumsy.
Tabby in Escape Room (2017), who has to count to work out that 'E' is the fifth letter of the alphabet.
Ghostbusters (2016): Kevin Beckman. Bless him, but as gifted as Kevin is physically, the pendulum swings hard in the other direction when it comes to his brains (specifically his lack thereof). He wears glasses but took the glass out so he didn't have to clean them, thinks covering his eyes blocks out sound, cannot figure out how to work a phone, only announces appointments when the person has been waiting for at least ten minutes, and wanders off during the final battle to go get a sandwich. Erin wanted to hire him because he is just so damn pretty, while the other girls went along with it because he was the only applicant.
Good Burger has the main character, Ed. When Dexter says he looks familiar, Ed isn't sure why but speculates that he (as in Ed himself) might be someone famous, like a baseball player, or a pretty nurse.
Tiffany in Headless Horseman. After Doc discovers that the 'blood' on her forehead is really red nail polish, her reaction is an astounded "Oh! So that's why it didn't hurt!"
The whole future society of Idiocracy is comprised of these.
Karen in Mean Girls Has a fifth sense, which she refers to as ESPN, where her breasts can tell when it's already raining.
In My Favorite Martian, after Martin clones Brace's body and mind, and later returns to normal.
Martin: [clutching his head] Whoa. Her head was dark and empty.
In Stranger On A Train, Bruno's mother is quite clueless and dotty to everthing concerning her husband and her son, Bruno. She is told her son may have committed murder, and continues to be happily unaffected and disconnected. One interesting note - she shows Bruno her latest painting, and the image is surreal and disturbing. This indicates that Bruno inherited his psychopathic nature from his mother, who is too dense to present any of the symptoms herself.
Not Okay: Danni starts off as this, somehow thinking that her article is worth publishing despite her saying things like she "missed" 9/11.
Sorina from Pain & Gain, who easily buys Daniel's story about him and the others being CIA agents and even seems to still believe it when testifying at their trial.
Dean Walker's daughter in Promising Young Woman who falls for Cassie's story hook, line and sinker and is left sitting in a diner convinced that her favourite boy band is going to show up to shoot a music video. Even Cassie, who is generally sympathetic to other women, tells Dean Walker that is a good thing that the girl is pretty because she isn't very smart.
Sharon in Satan's Cheerleaders, who has it to have explained to her what an 'unsoiled maiden' is.
Sheila in Scarecrow Slayer. Upon arriving at Caleb's farm, she asks "What is this? A farm or something?", to eye-rolling from Mary in the back seat.
Officer Ed from Scotland, PA. Why would anyone trust this man with a loaded gun? They don't.
Ed: Sir, I called the number, like, 95 times. I practically had it memorized.
Zangief in Street Fighter. His proposed solution to seeing, on a TV screen, a vehicle loaded with explosives heading toward the building he was in at the time was to change the channel.
Margalo from Stuart Little 2. She puts on an understated version of this, mostly to throw Stuart off the fact that she's an Artful Dodger.
In This is Spın̈al Tap, none of the characters are particularly bright, but nearly everything Nigel Tufnel says is, as David St. Hubbins puts it, confused.
Marty: Why don't you make ten louder and make ten the top number and make that a little louder?
Nigel:(long pause, staring at his amp and thinking) These go to eleven.
