Being ignorant isn't the same thing as being stupid. If you are very ignorant you might seem stupid but it is two separate things.
I don't think the US population has an above average percentage of stupid people, it might have an above average percentage of ignorant people. Then again this will depend on what you define as important knowledge. It seems as though many americans have very little knowledge about other nations, geography and international politics. Unfortunately all of this has come to the fore in the socalled "leader of the free world" which seem to be holding his incredible ignorance about everything before him like a banner.
In general is seems like the bigger the country, the less is the interest in what is going on in the rest of the world.
There are some really funny and quite caracteristical common phenomenons that you encounter with many US citizens and that is their habit to declare themselves "proud" of the strangest things. Like "proud" of being an American which for most is a total matter of chance. How anyone can be proud of being of any nationality is beyond me as you usually don't have much say in where you are born. Another common and quite irrational statement you often hear is that "I love my country, it's the best in the world, I wouldn't live anywhere else"
Such statements might not be a sign of stupidity but they sure make people seem extremely arrogant and silly.
As most of the US population don't even own a passport and some haven't even been out of the state they live in they do of course not have any idea whatsover of how it is to live in another country and which one is the "best in the world". I can assure the US participants here that the US is nowhere near being the best country to live in, it is far down on that list both statistically and otherwise. That said I wouldn't have any problems living in the US, it is not a bad place to be, but there are so many countries that is a far better place in most respects that counts for it's citizens. I mean just to mention one, how about free healthcare?
Travel is usually a very good way of discovering such simple and pretty obvious facts and actually living somewhere else or spending longer periods in other countries will soon teach you a thing or two you might not know.