It's one of those things that can go both ways. It also depends on how or what one wishes to consider "crazy". In the Breakfast club, there's a quote " We're all a little bizarre, some of us are just better at hiding it.,"
If you want to look on the side of mental illness, it would depend on what mental illness or disorders are present. For some it helps them embrace their writings natural flow. Some writers with anxiety disorder, especially those who use it as a coping mechanisms, tend to be more creative then some.
What you've experienced in life could also determine creativity. I've known kids whose parents discouraged creativity. Others are encouraged to let their imagination fly. Authors who have experienced a lot of rough antsy times may write the most creative angst stories.
I guess what I'm saying is that, in some people, yes it helps. In others may not be as much so.