I don't tell everyone that I meet that I'm autistic (on good days I used to be quite 'good' at masking). I've had a couple of people who don't know I'm autistic say to me when discussing the unreasonable behaviour of family members 'I think they're got Aspergers', which I don't think they would have said to me had they known of my autism.
They are nice people though and are neither "shits" nor "thick".
I think because I've so often elicited a bad response from people because of my autism (being accused of being weird or stand-offish for example) I hate to see others written off as bad people.
Sometimes people won't understand the implications of what they're saying, or will say the wrong thing and I don't think that automatically labelling them ableist is helpful.
The difficulties I have because of my autism and my exhaustion from being judged for them means that I rarely leave my house these days. There is so little print space given to understanding autism and educating people as to how it can manifest, so to me criticising a few individuals for careless words feels like the wrong focus.
At the end of all that, I'll sum up by saying that no, autism shouldn't be used as an insult, but it is something that I have rarely come across, whereas horrible behaviour towards people who are autistic or seen as 'different' is relentless.