Not vocaly but by actions yes.

Generally unlikeable people have, as did my parents. Haven't been told that by good people, which mostly give me quite positive feedback. Oh, and animals seem to enjoy my company.

About the only time I've been told I'm unpleasant to be around is if I'm having a meltdown or highly agitated (so near a meltdown).

The average person whom I mask around never sees an issue.

I'm great to be around! As long as you are neurodivergent.

I don't think of myself as a likeable person, but I don't really have any evidence for that beyond my own assumptions. If anything, the opposite seems true. I work in the service industry and meet new people every week, and most of them seem to like me, which I find a bit surprising. It's not that I think I'm mean or anything, I just assume the weirdness I feel inside shows on the outside and that people judge me for it. It might be that what I experience as weirdness just comes across as authenticity. I'm pretty openly myself most of the time, and even if it seems odd at points, it doesn't come off as insincere. Maybe other people respect genuineness the same way I do.

My Mum used to call me miserable.

No, but it doesn't matter 'cause I'll estrange myself all on my own anyways.

Miserable, no; quiet, yes. I don't know what to do in social situations, so I just pay attention, smile, and participate only when I absolutely have something to say.

In groups of people I don't know well, I don't say much but offer encouragement and react supportively - people seem to enjoy that. I'm told I'm a "good listener" when that's literally just my defense mechanism. It's wild what information people will volunteer just to fill conversational space.

With people I know well or who share my hobbies/obsessions, I will be more likely to participate until I run out of energy. I've convinced myself (with no evidence) I'm annoying or have nothing to contribute, so being quiet minimises the damage either way.

No some people are insufferable assholes. Some are autistic, some are not. These things are not related.

Nope.

Edit: removed long comment as not very helpful for OP