The mistake people make is thinking Big Mom is a fighter, she isn't. She's just some overgrown child that was naturally a top tier. She hasn't honed a single one of her weapons to perfection, she's not efficient in battle, that's what Oda is trying to portray for quite some time now. Her natural stats carry her, and that's fine that's how she's made to be, her ability to absorb damage is insane, but that one thing alone can't gift you victories against other top tiers. We sometimes forget this isn't an organized boxing arena, Big Mom has proven she's a top tier and that's good enough, she doesn't have to be able to defeat the other top tiers.
It's a very different take from the Admirals who are well oiled killing machines, built /trained for battle.