Yet relatively few mental disorders, from my reading, seem to have direct effects on an epistemic level.
That sounds very strange to me. The standard things like schizophrenia, paranoia, etc. are characterized precisely by a "wrong" picture of reality. If a paranoiac is unwilling to venture out to buy groceries, that's not a failure of instrumental rationality -- if there actually were people outside his door who want to kill him (as he believes), his behavior would be perfectly rational.
Even depression has a strong epistemic component.
Sure, but I don't think I'd describe that as a deficiency of epistemic rationality. People dealing with disorders like paranoia clearly have unusually strong biases (or other issues, in the case of schizophrenia) to deal with, but exceptional epistemic rationality consists of compensating well for your biases, not of not having any in the first place.
Someone who's irrationally predisposed to interpret others' behavior as hostility, and who nonetheless strives with partial success to overcome that predisposition, is displaying better epistemic rationality skills than Joe Sixpack despite worse instrumental outcomes.
Another month has passed and here is a new rationality quotes thread. The usual rules are: