Ok this is really stupid, but for the life of me I don't get it. What is a Mouth Breather? Simplest definition is one who breathes with their mouth instead of their nose?

Is this supposed to be funny? an insult? What. I've seen that alot lately on the regular forums but I guess I don't get it if it's meant to be an insult or if it's supposed to be funny, in what way is that funny?