This is one of the few points where postmodernists (not the Judith Butler ones) are right about modernism: mental illness is mostly a social construct and just means whatever the Zeitgeit is at the moment. Retards and insane people were considered holy in the early Medieval because people thought God spoke through them, this went to "Retards and insane people are irrational people (or not even people anymore, animals) and are the enemies of God and should be kept away from normal people" in the Renaissance to "they're small children" in Freud's era to "they have the mental capacity of children and need our protection" today.
In the last 50 years we went from "homosexuality is a mental illness" to "traditional masculinity is a mental illness". Both of these claims would be rejected by true postmodernists, since the definition of mental illness is very obscure and changes all the time. Go 2500 years back to the Greeks and they wouldn't even now what you're talking about. They had a concept what we could call mental illness, but it wouldn't make sense to them why somebody would consider "homosexuality" and masculinity a mental illness, since for the Greeks illness had to be expressed in actions and your emotions (for example: you're too lazy, too sad, too optimistic, too pessimistic, too aggressive, too shy etc.).