How many people did they interview? How many questions did they ask each person? Were questions asked in the same way? Once someone answers too many questions correctly do they move on to the next person?
Scientifically I don't think this video could prove that the general American population is stupid. It does prove that people can be found and they are more than willing to answer questions with incorrect answers.
It is not hard to say, "I don't know." Intelligent people don't answer the question if they don't know the answer.
I think it is too easy to take a camera to someone and when they say, "I don't know," to a question, make them guess and they will sound like an idiot no matter what the guess.