This is one we could go around on forever, because there IS no answer. Sure, some people might only produce one great work and then commit suicide. That's not proof that being depressed produces the greatest art. I think that there is an entropy that colors the way we value things. Comedies rarely win awards because they are considered somehow "less" than drama. Comedy is thought of as lacking in substance. Like everything else in life, great art is produced as a product of just about every emotion.
There are plenty of writers and musicians who just seemed to have one or two good pieces in them. To me, Call It Sleep is a great, great book, but Roth never produced another work of such brilliance, and went something like twenty or thirty years without producing anything.