There are a few theories on this. I personally thought Miyamoto looked in a Japanese/English dictionary for a translation of the Japanese word for stupid/goofy.
Which I don't think DK is, btw. Stupid, that is. He has something of the noble beast about him. I always play him at multi-player N64 games. A very fearsome prospect at Mario Tennis, I can tell ya.
Oo oo oo! I'm in a monkey mood now. Gimme banana.