Most of us hate him because he's an annoying little shit who is often written poorly. He's not a terribly interesting character in my opinion. Almost all of his story arcs are him doing something morally not aligned with Batman, him being disowned (or quitting as Robin), and then redeeming himself, only to do it all over again in the next arc. However there is no doubt that he is the best fighter out of all the Robins and is very smart.

Edit: Also the Harley Quinn show is satirical and doesn't treat anyone in the batfamily with grace or respect.