"Do people really expect mentally ill people to wallow in self pity and not do anything when they're signed off?"
I don't think that's it, GerddwrEryri. I think (in the workplace, anyway) it is more a case of people expecting all people who are signed off as being too ill to work, to also be too ill to party. Regardless of whether that illness is physical, mental or a combination of the two.
In fact, it would probably be helpful in your case, if people did judge, did differentiate; and so differentiated between mental and physical illness when judging whether being too ill to work, is also be too ill to party. In these circumstances, people may well then see that it can be helpful (in coping with one's illness) to socialise and keep in touch with colleagues, rather than have the kind of isolation that one would have with, for example, 'flu.
Mostly, though, I think it's likely that in the workplace, colleagues would expect everyone who is ill to stay at home, until better!