بسم الله الرحمن الرحيم
In common situations, it is sunnah for each one whether he is fully healthy. If the said person is suffering from mental illness and it is feared that he cannot maintain the marital rights then it is not necessary to get him married.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.