According to an article on Good.is, a happy writer is a bad writer.
It's a good headline-grabber, but the actual article states that a low-level bad mood has a positive effect on people's ability to detect lies, see past stereotyping and think clearly. The kind of bad mood that lasts for a short while, not that proper bone-deep misery that makes it difficult to put the kettle on.
I think this makes sense. I find it very hard to write from the very hardest and darkest times on my life (I do keep trying though, because I suspect the really rich material is there. Next time you see me crying, say 'rich material, though' and watch me cheer up. Or swear at you. It's the uncertainty that makes me fun, honest). The little bad moods are easier to play with - mild jealousies can be filtered through a character and become life-changing, for instance.
The stereotype of the unhappy artist bothers me, though, largely because I've never met one. I've met unhappy people who happen to be artists, but I've never met an artist who wasn't happier as a result of being an artist. Even in the darkest of times, most artists I know can think 'well, at least I can write/paint/sing/do interpretive dance.' The stereotype is that the type of person who becomes an artist is also drawn to misery, and I'm not convinced. Artists seem to be drawn to experiences, sometimes extreme experiences, but they do spend most of their lives following a light, which makes a difference.
That said, it is comforting to know that being a low-level grumpy cow might pay off for me. So far, all I have to show for it is a premature frown line :)