Is it permissible to pray behind someone who claims imamate, but is mentally ill and none of the scholars have approved of his imamate?

If the Imam's intellectual deficiency does not reach the level of insanity or idiocy, then his prayer is valid, and it is permissible to pray behind him. This is because the general principle is that whoever's prayer is valid for himself, his imamate is valid for others. It is permissible for a less virtuous person to lead the prayer even if a more virtuous person is present. One should think well of the Imam, and it is not permissible to have ill thoughts about someone whose righteousness is apparent. There is no objection to seeking a better Imam to pray behind; otherwise, do not abandon congregational prayer behind him.