Except that it's quite not true that all people are dumb, ignorant, and bigoted. Dumbness (or more precisely intelligence) has a distribution over the population, and selective ignorance and bigotry in certain ways is correlated with politics.

In particular, eligibility for work visas actively selects for non-dumbness: you have to get hired for a job. Eligibility for birthright citizenship does not. So there are solid reasons to believe that immigrants to a country (any country) are statistically less dumb than native citizens.

Immigrants also necessarily have more life experiences and more personal knowledge of people of different backgrounds and origins, which makes them less likely to be ignorant and bigoted, too.