I thought people were using it as an expression that wasn't intended to be taken literally.
eg. Jake's so mental, he parties hard every night. It doesn't mean he's actually mentally ill.
In the same way, saying I'm so OCD about xyz doesn't, for me at least, mean someone's actually trying to convince people they have this mental health disorder. If my flatmate said to me "sorry but can you tidy up after yourself a bit more, you know I'm a bit OCD about the kitchen being clean" I would assume she was using this in a figurative sense, almost like an expression, rather than informing me of a genuine health issue and/or attention seeking.
That's just my interpretation though, of course. I can see how this exaggeration/misuse of the term is inappropriate and offensive to those who have genuinely experienced these issues, too.