Alias Hillbilly Teeth. (Google that phrase to see wonderful examples better than any lengthy actual description, although mixed with British Teeth.) A character having very protruding (upper) teeth always immediately suggests that he's one bat short of a belfry. The canonical example, unfortunately from Germany and thus less known, is the roles of Norman C÷ster in "Bernd das Brot". Overlap with British or (less probable) Chinaman Teeth is possible. Surplus Speech Impediment is for free. Trope needs, first of all, examples. Seen It a Million Times.
- Loriot's "Monster Actor" sketch. He certainly is a slow burner ("Mask? What Mask?") but these rather are monster teeth.
- Cuthwald from 'NSFW Comix'' falls rather under British than Crazy Teeth, and is more leech than loon.
- The Simpsons. Everybody in the Simpsonverse has an overbite (and granted, nearly everybody is rather stupid) but e.g. Cletus - who would be the candidate for this trope - isn't really different since it's the artists style.
Hello, Unknown Troper. You'll need to get known to lend a hand here.