I think that would be much too difficult to generalize as a blanket statement since there are plenty of truly extraordinary musicians who are well balanced and about as far from mentally ill as you can imagine. Just a few off the top of my head would be Itzahk Perlmanm, Placido Domingo, Paul McCartney. There are tons of others.
And there are tons of mentally unstable people with no musical (or artistic) ability whatsoever.
But I do remember a news magazine feature on people with Williams Syndrom. This is a chromosomal abnormality with distinguishing characteristic of a high correlation with both extraordinary musical ability combined with severe mental deficits. An abnormally high percentage of those with Williams syndrome are born with perfect pitch and have a tremendous affinity for music.
There are also other concommitant characteristics both physical (elfin appearance) and personaity wise (extreme sociability) combined with severe mental deficits regarding functioning in real world situations and impaired judgement.
It was really a fascinating report. I believe it was on either CBS (60 Mins.) or ABC. can’t remember which but I’m sure you could find it.
So, in specific cases there can be a definite correlation between the two but not as a generalization or “rule of thumb”.