I don't know about stupid so much as generally ignorant - by which I mean extremely self-centered - and very loud and obnoxious. Obviously that's a stereotype but it does apply to more people and places I've been to in the US - as well as Americans coming here - than I've ever experienced anywhere else. I also think that perception is coloured with a bit of jealously too. Most people in most other countries aren't nearly as self-assured in that "manifest-destiny" sort of way that Americans are either. There's definitely a bit of envy in there. There are times when that sort of arrogance is insulting, and other times it's pretty admirable. Let's just say they definitely make the world a lot more interesting for both the good and the bad. I know I think it'd be a much duller world without them, and given how much time I've spent in the US I certainly love them more than I don't like them.
“There is no such thing as a weird human being. It’s just that some people require more understanding than others.” - Tom Robbins