disabled physically differs from disabled mentally. People who are disabled mentally to an extent where they need someone to care for them constantly are advised to not get married, or their caretakers are advised in this case. ( i think i may have heard that it would be haraam or makruh for people with an extreme mental illness, as it would do more harm than good, but I'll leave that to the pros of shiachat to determine) This is pretty logical, 1) the spouse would be more of a nurse than a husband/wife 2) if they were to have children, the illness could be passed on and then you'd have more people to take care of and 3) it could be argued that someone with an extreme mental illness "doesn't need to get married" meaning they do not seek the companionship those with a sound mind would. Furthermore, I'd like to mention that i have heard of and known couples that are mentally disabled to a mild extent. They seemed to be doing well under their parents' supervision. If they are able to, then I don't see why not.