Some people with autism are severely mentally retarded. In fact, classic Kanner's syndrome (the original autism) generally also means severe mental retardation. Other people are labeled "high functioning autistic" (also called "Asperger's Syndrome") and can be very intelligent but have some challenges in areas like social skills. Still, most their social difficulties are, essentially, a form of being mentally challenged. Though there are some exceptions. For example, the folks who are also faceblind (fairly common among autistics) face social challenges due to their difficulties recognizing people, which is different from just not understanding the social stuff but has a similar outcome.
(Both my kids fit the profile for ASD. No formal diagnosis.)
(Both my kids fit the profile for ASD. No formal diagnosis.)