firstly, you can’t “look” autistic.
Autism is a mental condition and doesn’t affect you physically. That means anyone, regardless of looks, can be autistic.
Secondly, that was pretty insulting to all the autistic people.
Saying you “look autistic” implies that you believe that all autistic people look a certain way. Many autistic people are already being doubted by their close ones when they say they’re autistic since other people go “but you don’t look autistic.” To add insult to injury, this comment implies that autistic people all look “dumb” (I wish to refrain from using the word). That’s a really bad generalization as not only are autistic people being doubted as I have already mentioned earlier, people start associating autism with ugliness.
Why is this a bad thing, you may ask? I’ll be splitting this point into two tiers: how it affects autistic people and how it affects people around them.
Onto my first tier on how this idea affects autistic people. This causes autistic people to hate themselves. When you associate autism, something that a person is born with and cannot change about themselves, with ugliness, it implies that their self-worth is less than that of a normal human being, that because of their mental condition they are ugly, even though that’s something that is completely out of their control. In the best case, some are hardly affected by this, but in the worst case, autistic people take the depression route. (You know the drill.)
Onto my second tier on how autism=ugliness also affects people that are not autistic. This causes ostracism for the autistic people. When people without autism associate autism with ugliness and see that they are superior to autistic people, the ostracism begins. Bullying can be due to a variety of reasons, and one of them is empowerment, the feeling of superiority over another. Soon enough, if you have too many kids thinking that way, the autistic kid is going to have a really bad time (megalovania theme song plays) and spiral down the depression route again.
Tl;dr, don’t use autism as an insult because it hurts. It’s not that hard to not be a prick.