what is the disability though? is it of the body or mind? something like a lame leg it's shown there are fixes in equestria. artificial limb or healing magic. but mental? I'm not sure that's much of a thing
ponies are very simple minded on average, my guess is less can go wrong when it comes to the mind of a pony. that and the fact that equestria is a near perfect land of magic and friendship. these things just don't exist. asking "but what if?" ignores the point that they don't exist because fundamentally they're not really possible in equestria the same way they are here. "what if" for mental illness to exist in equestria like it does here they'd have to have such other terrible things as we do. and for that equestria would no longer be equestria. making the point and question pointless

Depends on which writer is pandering to the executives.