Absolutely not! I've met quite a few American scientists, and they're brilliant at their work. There are a lot of intelligent, cultured American writers and musicians. I certainly don't think all Americans are stupid.
However, there are a lot of ignorant Americans. Ignorance is not stupidity - it's merely a lack of knowledge. You have a lot of Americans these days who don't care to learn about the world or their own history, and in some cases, seem to be downright PROUD of their ignorance. Even worse, there are a lot of Americans who sneer and look-down upon people who are knowledgeable, calling them "elitists" or "intellectuals". Since when was intellectual ability a bad thing?
This is why US universities are having to rely more and more upon students from overseas. America has some INCREDIBLY intelligent people, some of the world's BEST scientists and institutions..... but the culture is turning against them. Scientists are ridiculed as "Eggheads" or "Nerds", whereas "bad-ass" rapstars and basketball players, who have never invented or really contributed anything to society of material worth in their lives, are venerated and applauded.
America used to respect science and technological achievement. It used to respect those who studied hard and used their imagination. It used to actually listen to what educated people had to say. Now it doesn't. Ignorance is seen as a virtue. Education is seen as a suspicious "values-warping" threat. And anyone's opinion, no matter how IGNORANT must be "respected" since of course, "what do the scientists know?!".
Americans are not stupid. But they are ignorant and they are neglecting science and mathematics. You are one of the only developed regions on the Earth in which the majority believe in Creationism. The majority of your teenagers can't even name the past 10 presidents (whereas I can, and I'M NOT AMERICAN! Obama, G.W Bush, Clinton, Bush senior, Reagan, Carter, Ford, Nixon, Johnson, Kennedy, Eisenhower, Truman, Roosevelt, Hoover, and I can go on all day. I even know what they are famous for) and know next to nothing about their history. Americans forget things that happen in their own country so damn fast. The Financial Crisis and Big Bail Out of the banks occurred under The Republicans in 07, yet a substantial amount of Americans have COMPLETELY forgotten that fact and now blame Obama for the bad economy. The Republicans controlled the economy for 8 years and destroyed it, and now you blame the guy who comes next?
I don't view the Americans as stupid - well, at least, no more stupid than any other population on the face of this earth. But they are wilfully, almost proudly ignorant. They hate facts. They don't want to read up on their own history (or at least, they don't want to read a book that might disagree with them and shatter their own illusions on their nation's history). They no longer respect Science or Mathematics - which are the KEYS to your military strength. Your missiles and your tanks and your advanced radar and your jet fighters are designed BY SCIENTISTS AND ENGINEERS, USING MATHEMATICS! Those weapons don't fall out of the sky! They are made by people who studied hard and who used their minds! And you don't respect them! You don't fund them! You don't care about them! You mock their knowledge, you disregard their opinion and you despise them for disagreeing with you, despite ALL they have done for your nation.
America is still the technological powerhouse of the world, but that is changing! It's changing! I know plenty of scientists who are leaving because they can't get funding in the US. I know of labs being shut down because your government thinks building a sports stadium is SO MUCH MORE IMPORTANT than funding, Oh, I don't know, a CANCER research lab! You'll spend hundreds of millions of dollars venerating and celebrating the exploits of "Snoop Doggy Dogg", who has never done a damn worthwhile thing in his entire life, and yet give a pittance to a charity that wants to advance the cause of science!
The fact that people value trash like Lady Gaga over a Nobel Prize winner means that your nation is going downhill, and fast. Which is a shame. I like America. I really do. I plan to visit or work there one day. I love your constitution, I love what you have accomplished, and you have accomplished a LOT. But now, you are throwing it all away, and you are doing it with your own two hands.