It's kind of a running gag. In the game, he was designed to be obnoxious, annoying, and generally disliked by everyone. He insults other monsters, doesn't fight, does nothing but complain, and just makes things seem to drag on (He literally just makes the combat phase longer).

He is so disliked that every enemy you fight will stop trying to kill you long enough so that all of you can ditch him. And he STILL finds a way back even when it's clear nobody wants him.

And he is the only character in undertale who isn't given a single redeeming factor or likable trait. If you can think of one, please do tell.