Zig-Zagged: Sometimes Bob is presented as stupid, sometimes of average intelligence, and sometimes a genius.
Averted: Bob's stupid, but still vaguely capable of carrying out normal human activities.
Enforced: "We need a character who makes even the stupidest characters look smart by comparison."
Lampshaded: "Bob isn't the brightest crayon in the box."
Invoked: Bob stands under a coconut tree and a very large coconut lands on his head.
Defied: Bob keeps his smarts up any way he can.
Seeing that Bob has absolutely no concept of morality or consequences and that nobody would ever suspect him of anything, Manipulative Bitch Alice convinces Bob to play a "game" with his friends involving a shotgun. Bob then spends the rest of his life in a mental hospital, with no idea why his friends won't ever come back.
Reconstructed: To prevent him from being manipulated into harming anyone (including himself), Bob is always accompanied by his guardian/attendant Carol. Carol lampshades all the bizarre stuff Bob does and says, and - due to being constantly by his side - probably has a minor Butt Monkey status.