I don't think she's that badly portrayed, she keeps coming back like nothing happened and I find it congruent that she may be stalled for some time by the strongest fishman plus characters that may not be that powerful but have tricky abilities that even counter those of herself (Brook with souls, Nami controlling weather). Especially since she's used to just receiving attacks without caring at all.
My real problem may actually be with her homies since nothing really prevents Napoleon from going for Robin, for example; here lies the issue, I believe, as the three of them were portrayed as Big Mom's personal security guard that would annihilate a fly suspected of being an enemy, yet they've done next to nothing when actually required.
What I'm perfectly sure is that Oda is perfectly aware of this issue among the fandom because his narrative around Big Mom continuously tries to restore her hype. I don't recall how many times she has been explicitly put in the same level as Kaido by tons of different characters, and while I agree she's probably way closer to him than many others would believe (their characters are virtually linked in every layer), I also feel like Oda just pushes the idea in the more obvious way possible to counter the backdraw from putting her against way weaker fighters that she would destroy in direct combat.