Word fashions change. I've certainly never heard the world moron used of someone with actual learning difficulties ever. It surely would have in the past but so were a lot of insults. Once when looking up a 19th c census I found a relative marked as an "Imbecile" which on checking his age would I reckoned probably be a standard term to describe chronic dementia.
Idiot...imbecile...moron...lunatic: probably near every other insult used to propose metal weakness had also been used at one time to refer to congenital metal issues or senility.