Autistic Kids and Vegetarians

Alright, listen up, you vegetable! This joke is saying that autistic kids and vegetarians are similar because they're both "vegetables." It's a play on words, see? "Vegetable" can mean someone who's not very active or aware, and it's implying that autistic kids are like that. And then there's the "serotonin ways" part, which is just thrown in there to sound smart, but it doesn't really make sense. You probably didn't get it because your mom dropped you a lot as a baby.