Hitsugaya is hated because his popularity is directly influencing the manga. He's being shoved into places for no reason other than to please the audience (which can work sometimes, but not in this case).
Did I think he was cool beforehand? Yeah, sure. He was another awesome character Kubo managed to create, but unlike the others, who are more deserving of a little spotlight, Hitsugaya is shoved down our throats like so much splooge.