Even though autism diagnosis and therapies fall under the realm of psychology/psychiatry, autism, including AS, is not "mental illness" but atypical neurology resulting in a specific complex of symptoms and traits. When autism can cause severe developmental challenges and delays, it doesn't qualify as a mental illness because it does not necessarily cause mental illness, i.e., a psychologically impaired state. In fact, it is thought that there's a many high functioning autsitics in the world who are not dx'd because their autism has not caused them any significant problems. Autistics can, however, develop secondary mental health problems, like depression, anxiety disorders, PTSD and even personality disorders, just like neurotypical people.
I took teh MMPI too when I was young, and while I was not officially dx'd with a personality disorder (I was too young - PD are normally only dx'd in people over 15). There was concern about my "weirdness" though, which the MMPI results clearly illustrated. They couldn't figure out what was "wrong" with me - but that was years before AS as an accepted dx in the US, so they simply didn't have a dx that fit me. 