It’s mainly Luffy and Zoro fans that hate him so much from what I see. Luffy and Zoro are two of the most popular characters in the manga so it will seem like a lot of fans hate Kidd because of how large their fan bases are.
- Luffy fanboys hate him because they are insecure about the author portraying him as Luffys rival. They feel that Kidd doesn’t “deserve” to have that position in the story. Luffy fans probably felt the same way about Law before Dressrosa happened and Luffy proved to be way above Law. That has yet to happen for Kidd yet so Kidd is still a threat to them for whatever reason.
- Zoro fanboys hate him because Kidd is what they think Zoro is. You see, many Zoro fanboys have this delusion that Zoro is equal to Luffy in strength. This is obviously insane but don’t tell them that lol. Kidd is actually the one in their same generation who has that portrayal to be Luffys rival/equal in individual strength so Zoro fans obviously will try to downplay him at any chance they get. It doesn’t help that Kidd is almost featless so it makes them even more mad that this magnet with almost no feats is being put above their precious Zoro