I don't believe you can generalize a whole country under a single adjective, be whichever it may be. Now, calling a whole population "stupid" is even worse. I think there are, indeed, a big amount of dim-witted people out there, but there's also bright people nearby.
I think it's not as much as stupid people but what you ask of your people. If you start imposing policies that make you limit your creativity and development in the name of "safety and fairness", you are asking to dumb it down! And besides, if you give people the choice to not having to think too much, most will take it. Humans prefer pleasure before work! So in a first world country where there's a lot of shortcuts and easy paths, there's bound to be people who are used to be breast-fed every single thing and thus will have trouble when having to actually thing for themselves.
I do not like some of the american policies, such as the zero tolerance, but I do love some americans! Mooshi, you oughta know that better than most!
And there's Karizma, and Rab, also from Texas <3 and a BIG bunch of other people I hold very dear!
So no, i don't think "American's are stupid"