In a social species, stupidity is necessarily damaging to others.
The point of a social species is to exploit the gains from cooperation. These are so powerful that social species can totally dominate nonsocial species, with the result that most reproductive threats will come internally, not externally. External reality is dominated by the subset social reality.
Cooperation means talking, which means influencing others, and the cooperation means this influence is more important for resource allocation than any other factor. The influence of the stupid is necessarily a net loss.
The primary example is that people like me can convince the stupid to serve my interests instead of their friends' interests. In extremis I can whip up an army of stupid people and kill other, more responsible, non-stupid people, at little risk to myself and at great risk to the stupid person's friends. Even assuming these friends aren't as stupid, they made a mistake investing in friendship with an idiot.
Being stupid is a crime. To be stupid is to defect against society. It's not their fault, but nevertheless they must be punished and excluded for society - the social cooperation unit - to be healthy. For example, by being enserfed and disenfranchised.
Notably I think since public choice theory is true, eugenics is impossible. However, serfdom could be a requirement for welfare - when the truly stupid risk starvation due to their stupidity, they will willingly trade their freedom for their life. This would formalize the command relationship between the intelligent and the stupid, meaning society can easily trace the true responsibility and hold the responsible accountable. Alternatively, the idiots starve and stop being a problem.
In modern times, formalizing the command relationship is gauche, because fashion flows from the elite, and the elite have realized they can evade responsibility by discouraging the hobbits from letting commands be formalized. Women love it too - nothing more dominant than evading accountability and thumbing your nose at your victim/followers.
This is one of those times it would be pretty cool if someone proved me wrong.