Why some people think im mentally challenged?

Mental disorders are a common way of people dismissing a topic they don't understand. It's basically a defense mechanism, so something about you threatens their ideas or beliefs and they don't want to risk damaging the credibility of their views

Maybe the thing that threatens them could be a legitimate mental health problem, but more often than not it's just close-minded rubbish

And so what if you were ? - tell him that if he calls you that one more time you will register a complaint with the Americans with Disability Act. And sue the place for workplace violence / harassment. But only if you are - of course

Tell him to suck on a chicken leg.

Tell em to fuck off