It's not that Americans are stupid, that's an unfair generalization. However, we have a very large working class, and only 25% of the population even have a college degree. What makes this really sad is that our universities really aren't that difficult to get through. I can tell you from personal experience, that not only do working class students complain about their education costs (which is actually affordable), the vast majority of them are very ignorant and religious. They flood the universities inspired with the American dream that one day they will live a better life, force the universities to drop their standards to accommodate the influx of stupid people, and then fail out financially broke. To give you an idea of how stupid working class people in America are. I was the only upper middle class student in a class of 30, and none of them were able to answer the professor's question of what the Cuban Missile Crisis was. I decided not to answer (correctly) for several seconds but I saw all puzzled faces. In that class I also gave a speech on real politik, assuming they would know who Otto von Bismarck, Metternich, and at least Henry Kissinger were, but I only had puzzled faces. My grade was dropped as being inappropriate for the audience - I over estimated their intelligence.
On the other hand, upper middle class Americans take their education seriously, and are able to reason abstractly. It comes to no surprise to me that most the high achievers, people pursuing science or other academic careers, came from upper middle class families in my experience. Furthermore, they are able to reason abstractly, and discuss intellectual topics.
I don't know how large the working class in Europe is, or how ignorant they are. But I can tell you that my upper middle class Danish friend said that working class Danes are also stupid. Though the US has a large working class, it is still unfair to represent all Americans as being like them. Upper middle class Americans are very different, as is the quality of our education. You should look at America not as a big lump of ignorant people, but as a country with an excess population of working class citizens. You should also consider that each state has unequal education. For example, South Carolina is pathetic, while Minnesota (which is culturally Scandinavian/German) has one of the best public education systems in the nation.
My point: Working class people are stupid everywhere, and the quality of education substantially differs state to state.