It’s a fair criticism but imo it’s not too hard to write off that bit of absurd narrated exposition at the beginning and instead think of it as being about anti-intellectualism and broken institutions, since the rest of the movie seems like more of a satire of those things, and the plot doesn’t really land on eugenics as a solution to its problems.

Mike Judge made a statement about it in an interview:

We live in polarized times, which means that for every ‘Idiocracy is a documentary!’ take that goes around, there’s also a critique that argues that Idiocracy is a celebration of eugenics. Have you thought about that interpretation of it?

Well, yeah. I mean I’ve actually read that a couple of times. But to me, I thought the opening made it very clear that whatever side you take, nature and nurture are both covered in that. That guy is clearly not a good father. I mean, there’s a kid with a motorcycle in the front yard, and no one’s paying attention to this. He’s just irresponsibly knocking up different woman, and proud of it. It’s not like he’s a good, upstanding role model for the kid. So I think it’s pretty clear here that, whichever one it is, [nature or nurture], there’s some combination of both. I obviously don’t believe in eugenics. I think you could look at it both ways—you have this couple that’s trying to be so responsible that they end up never having kids. Then there’s another couple who just irresponsibly keeps having them and not raising them right. So, you know, if the other couple adopted the other kids. I’m sure they would probably be better off.