I’m not SUPER well-versed in this because I myself do not have a learning disability, but here’s a good explanation:
http://disabledfeminists.com/2009/10/11/ableist-word-profile-idiot/
Although some don’t consider it ableist, a lot of people with mental disabilities are, and for that reason I’m teaching myself not to use it because I want all the people around me to feel welcomed and comfortable. ouo