Idiocracy is sort of like the Left's version of the Great Replacement Theory. And the underlying premise doesn't work for the same reason that the Great Replacement Theory doesn't work.
That being said, Idiocracy's predictions have been scarily prescient. People are making dumber and dumber decisions. But it's not because of dumb people having more children than smart people. It's more due to economic inequality, parents failing to teach their children to think for themselves, people being exposed to more and smarter propaganda, and perhaps things like childhood malnutrition.