I think there are many great artists who are mentally ill. And I think there are many great artists who had happy, stable home lives, never felt depressed a day in their lives, and generally just have a good life. And many mediocre and very bad artists who are either mentally ill or not.
And then there are the mentally ill who would be good artists if they didn't give in to their mental illness completely. I know one particular person in my life, a schitzophrenic, who has done that. She stopped drawing about 15 years ago because it made her "nervous." Then she gave up sewing (she's an excellent seamstress). Then she gave up any books that weren't written by General Authorities. Now she barely ever leaves the house, and wonders why her voices are always so strong. She doesn't have anything else in her life to distract her, I think.
I'm not trivializing her struggle *at all*. She's had a hard life, and it's a hard thing to deal with. But continuing with something she loves *is* therapeutic.
That's off topic, I suppose.
At any rate, mental illness is just another factor. It can really help with creativity ("what are the voices in my head saying today?"), or it can really hinder it (clinically depressed who can't meet a deadline because they sleep all day). Or it can be in the middle--sometimes a challenge, sometimes a blessing.