-In WCI Big Mom was too powerful for Oda to deal with so he made her a mindless beast. Not neccesarily a bad thing, but the execution of it made her look awful. An endless succession of “Big Mom is about to kill everyone! Oh wait, nvm” so she looked awful and a joke by the end of it.
-in Wano Big Mom got shunted aside as a side villain compared to Kaido and she looked shit in comparison to him (plus continued gag moments) because she didn’t actually belong in the arc.
-Kaido someone managed to have a fight that was both far too long and rushed as fuck. So, yeah, he was really strong, no doubt about that, but the hints of an interesting personality were never really expanded on and in the end he was just not that great a villain.
So, ultimately, bad writing.