This is completely out of universe logic, but I think the only reason why Big Mom didn't seem impressive was because she had to lose to Kid and Law in the narrative. Big Mom was constantly referred to as being just as bad as Kaido throughout the arc, and was treated the same way up on the rooftop. It's not stated anywhere, but I kind of think she wasn't going all out against Kid and Law, especially considering she didn't buff herself with her devil fruit powers until later in the fight. And it's debatable as to whether or not she was even using advanced Haoshoku. She might have been holding back because she was so confident of her own superiority that she let it get to her head. And Kid and Law's awakenings were great counters to her without having to physically overpower her.
I'd probably still give this to Luffy and Shanks though. Luffy beats Big Mom with extreme difficulty. Shanks beats Kaido with extreme difficulty.