It is a joke.
Idiocracy relies on the assumption that the "more intelligent" people are snobs and too busy and too perfectionist with their life to have children, while the "less intelligent" people are mostly having sex and are too lazy to apply contraception properly.
Essentially, the first half of the movie is a justification for elitism.
I'm less afraid of Idiocracy to become true, but more afraid of politicians acting as if they believed in this movie.
The second half of this movie is more interesting, though. It may be interpreted as an analogy of how the most gifted people (in the sense of "intellectual giftedness") may feel living in today's world. (by putting today's "normal" people into a dumb world) But even that is, of course, a funny hyperbole, and nothing to take too seriously.