Banjos do have a lot more comedic potential than guitars--just listen to the opening line of "Flint Hill Special." Also for a number of years after the minstrel period and before the rise of bluegrass, only comedians played banjos (A good number still do - Steve Martin and Kevin Nealon for example). That said, here's another old one:

What's the difference between a banjo and a trampoline? You're supposed to take of your shoes before you jump up and down on a trampoline.

Jon W. (Aspiring banjo player)