I would not call Biden stupid. No one gets that high in the .gov being stupid.
I would.... the evidence strongly supports that conclusion. Biden was forced to drop his '88 presidential campaign due to plagiarizing other, campaign speeches, including easily recognizable ones from Kennedy. He also nearly flunked out of law school due to plagiarism. Plus, he can't spend a minute in front of a camera without saying something idiotic. While smart people can and do say dumb things from time to time, they don't continue to do so repetitively, without fail as Biden does. He certainly has some political skill, but he is where he is because he's reasonably likeable, was absolutely no threat to Obama, and because as a Democrat, intelligence isn't a requirement.