I don't think that's what the GP was going for... rather, implying that flat-eartherness is uncorrelated to IQ and, thus, the average IQ of flat-earthers is the same as that of the general population.

Flerfers seem to be a somewhat different problem. In my experience, the vast majority of flat earthers are trolls, pretending to be stupid for the purpose of angering people.

I'm sure there are some genuine flat earthers out there, and I imagine that their IQs do average near 100 (perhaps a little lower). But I'm basing that on a general understanding of how people come to stupid beliefs, rather than from observations of individuals, because I'm not sure I've ever met a flat earther who actually believed what he was saying.