In the past people believed in lots of stupid things like "god exists", "the poor deserves its situation and the rich has earned its situation", "hatred toward blacks, Jews, neighbor country, homosexuals, handicapped and many more", "kings are good and democracy bad", "the strong country has a right to invade the week country", "the rich are evil and the poor saints" and so one.
So I think the idea that stupidity has increased is an illusion. The thing in the modern world is more that the tolerance for stupidity has decreased so we discover it more. The sad truth is also that each one of us believes in lots of stupid things but we are more interested in finding errors in other when in self improvement. Like Jesus said, "and why do you look at the speck in your brother’s eye, but do not consider the plank in your own eye?"