I haven’t watched QI enough and I don’t know its Twitter to get a general sense of what they go for in terms of tone.
My guess is personality disorders is a somewhat meaningless term to most people unless they have direct experience. I can't really think of anyone who has publicly 'come out' with this issue. I read a lot of MH print and online media (and roll my eyes a lot...I'm quite attuned to media laziness) for my job and I'm not aware of any specific coverage, but this is not to say there isn't any. In that sense, PDs are not as commonly discussed - there is no real narrative about them compared to depression, suicide or body image. Not being a Star Wars fan I wouldn't get that joke. Sometimes any low hanging fruit is what comedy goes for, but I don't think that's the case here.