well, evolution isnt an intelligent force trying to make a species that lasts for eons, it simply favors individuals who produce offspring who in turn produce offspring of their own, the fact that morons are masters of reproduction as you say means they are, always have been, and always will be favored by evolution, its not a mistake because as far as evolution goes its not a bad trait
i dont think it would ever really get to a point like idiocracy, a certain percentage of the population will always have to be intelligent, otherwise the rest will die off, lol, the premise behind idiocracy relies on automation of everything that requires even the smallest bit of intelligence to do, so that intelligent people are no longer required for the population to survive, i dont think we will ever get to that point
BTW have you ever read the marching morons by cyril kornbluth?