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 Cöster 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.