I think the people seeing it as pro-eugenics are latching onto the “stupid person makes stupid babies” part as being the reason for the Idiocracy. Basically they think the movie is arguing “IQ is 100% nature and 0% nurture”

But IMO the intro pretty clearly showed that the main factor in the degradation of society is that the low IQ family were raising too many kids in a chaotic environment with no actual parenting.