previous poster's claim is a common piece of folk wisdom that ultimately stems from this paper: https://pubmed.ncbi.nlm.nih.gov/16298429/ 2005, n=187. the authors are blanchardites so the language they use is needlessly confusing, but they find 107.3 mean iq among the male-attracted trans women, 121.7 among the female-attracted
Being "mentally ill" and having the specifics of that diagnosis changed every time the DSM does due to social pressure (from homosexual/bipolar DSM 4 to schizo in DSM 5), I honestly think "mentally ill" has just become another loaded term. Most "menatlly ill" people are simply people who have used the faculties that have to choose to be abnormal. The reasons for that non-conformist decision are varied, but are rarely irrational. Generally, I've found that any non-conformist person, regardless the category of such they've been assigned, has a unique and valuable perspective at some level if you just get to know them. Sadly, that's rare in an age where everything new is threatening and everyone I don't understand is the boogey man. Protected groups do the same villification to those who practice societal norms. A genuine smile and a ready heart for other people that also bleed red and feel more pain than life should ever have allowed is probably a better start than pissing matches over who is right and who is evil.
TLDR - non-conformists are almost always interesting and brilliant in some sense, but they generally lack the ability to reach within the Overton window and effect change.
It's very weird how many absurdly smart people happen to be trans. It plays into the idea that mental illness correlates with IQ.