But they wanted more money.

And one piece was supposed to end with crocodile but they kept going because they needed more money. What bs logic is that. Yeah we know it's a trend to hate on dragonball(retarded but cannot ever be fixed) but saying that without any logic makes literally no sense.

Characteristic wise both kaido and big mom are badly built. They are just bulky dumb ooga booga characters that's all they had. Zero character development zero imagination. Even their fruits are just very strong but they aren't super expert with them like other villains in the past Katakuri,Doffy use them way better and smarter in fights. Despite that they are masters of Haki although from Big Mom side she didn't show as much as we expected.

Regardless of the hate One Piece is made because of Dragonball so Oda and especially him always has taken parts from many many mangas. Even the whole concept of retconman(gear 5) is based on UI revive which imo is a big one piece failure but anyways. It could have been designed much better but it wasn't the single thing of Wano that went wrong.

Like koming said Majin Buu and Big Mom have many similarities the thing is Majin Buu has more stuff than her because by absorbing character he starts getting their traits becoming smarter and stronger and keeping evolving. While he is very evil and he had no emotions he starts getting stuff and when he is back to his kid pure evil form he just wants to destroy everything(remember Big Mom when she went pure evil didn't thought about anybody even killed but didn't happen because of plothole Moscato?). But it doesn't stop there Goku's wish came true to reincarnated as a good guy(Uub) and having an entirely different character.

One last thing Majin Buu was two guys in one body the Good Buu(Fat) and the Evil Buu(Slim) when he appeared at first.

Because you mentioned Big Mom the amnesia was another wano plothole that made her immune to water despite the weakness of df abilities basically something to keep her alive because she would be designed to fell with a fight not from something so simple it would seem lame to see.

Also characters being dumb isn't something new in manga unless you haven't read anything else outside of one piece. For shonen it is an easy gag thing to break the ice towards a fight or anything.