There's an element of thoughtlessness (I won't call it stupidity) in the Trump phenomenon, and it's an element that exists in all countries and among the true believers in ALL political parties.
Most people (and I'm one of them) have insufficient knowledge of politics to stand above the mess and assess the issues analytically. The majority of those (and I'm NOT one of this lot) vote for the same party, year after year, in the mistaken, almost religious belief, that their party is best for them and perhaps even best for the country.
Trump and his publicists are expert at manipulating the mob to get what they want. If anything, his support says less about American "stupidity" than it does about the racism and bigotry of the average voter.