"Thanks, it's a constant act that causes me no end of stress and anxiety about the quality of the performance and leaves me feeling like my face is just a mask... but it's nice to know it's at least working."
I have no idea why anyone would ever think "hey you don't match my negative stereotypes about x" would be an ok and sensible thing to say, really with regards to anything, not just autism, sexuality etc.
if someone says this to you, respond with "And you don't look that neurotypical."
If they make an annoyed grunt, they are most likely not someone sensible. If they make a guiltful sign, than they realised how off that phrase was.
Fight fire with fire.