As an American it is horrifying to not be able to argue with this directly, because for every outward appearance it might as well be true.

But the argument I would make is that Americans are not the dumbest people in the world, Americans are simply allowed to survive and visibly prosper in spite of, and sometimes because of, their obvious stupidity. And combining this with the entitlement inherent in 'just happening to be born here' and the relative complete lack of suffering most of them experience, makes it easy for them to publicly hold opinions that people in most countries would either have to keep to themselves for their own personal safety, or just because few others are willing to join arms with them.

Basically it's a constantly building bubble that could happen anywhere, and to smaller degrees probably happens all over the world in small communities, but here it's a bubble that for some reason has been resistant to popping, to the point where any attempts to pop it are easily avoided due to it's mass and ridiculously protective userbase.

Look at the UK and Brexit, or Russia and the mass of people outwardly supporting Putin, or the Middle East and apparent support of honor killings. Even if the majority of the people living in these areas don't agree with this outward support, fear or resignation or something stops them from being the loud voice in the space. In the US it might be closer to resignation or hopelessness, but across the world we're all really the same when you sit down and talk normally...there are stupid people everywhere you look, here we just don't have a good way to embarrass them into shutting up.