Stubborn, you bet. Mean, you bet. Hold a grudge, absolutely. But never, NEVER stupid.
(of course, my family is from the country in the South and we actually know mules. I prefer horses because I like to be sure that my steed is dumber than I am . . . with a mule you're never sure.)
Never heard? Well that's what they were in the old days. I remember hearing that sort of thing from old movies and cartoons (Bugs?). Also, look at the "Swinging on a Star" song from the '40s - "...would you like to be a mule....he's just plain stupid with a stubborn streak."
Nope, in the old days they didn't hesitate to call them dumb. If they didn't move, they were "dumb" much more than "stubborn"!