Mental illness doesn't mean "crazy person", there's a huge number of such cases, and half the population or more is affected by one or more (20% on any given year).
There's nothing insulting or damaging to call someone as mentally ill (except for types who think it means "crazy person" or some such).
In fact, there shouldn't be anything insulting or damaging about being bodily ill as well!
The bottom lime is that autism is associated with certain impairments that affect our mental states (on the spectrum, have a close relative with another mental illness). It's not some X-men like new state of being that people should aspire too, but it's something we should respect, try to heal if we can, and learn to live with and support.