My answer would have to be 'it depends on the disability', but that of course applies to physical as well as mental disabilities. You might equally ask if someone who is totally paralysed but mentally sound should be disqualified from office, but a certain Prof Hawking would argue that such barriers can be overcome. Mental disabilities are challenging in different ways, and no mental illness is the same. The only test for all disabilities, mental or physical should be 'can this person still carry out the duties of the job in question?' They may need assistance and/or special equipment, but a blanket 'no mental disabilities here' is unfair and unreasonable.