The IQ of people unfortunate enough to be suffering from mental illness will follow the normal distribution for intelligence in exactly the same way as it does for people with no mental illness. However, there may be some tendency for those with extremely highly developed sensitivity to suffer in a repressive regime, for instance, or in the light of an understanding that is beyond the ordinary person.
Wonderful paintings have been produced by those suffering from psychotic episodes and suicide is perhaps more common in those with highly developed ability rather than those at the lowest levels of intellectual functioning