Tough question, and it would depend on how bad the mental unstable the person was. That is are they a danger to themselves or others? As long as they aren't a danger to themselves or others and the person doesn't want treatment, then they should be allowed their own creativity. Or as a manic depressive once put it as bad as my lows are, you can't imagine how high my highs are, and if she was happy with that who are we to disagree?
Some of the world's greatest minds are often mentally unstable, and their well being has suffered from it. Examples of this can include Stalin, Andy Warhol or Nikola Tesla, all of which might have benefited from psychology as we know it today. Assuming a link between creativity and mental illness in a way that forbids us from choosing "both" or "neither", what is better to preserve? Mental stability or creativity?