She's a waste of a Sweet Commander, and not only because of her strength. The biggest problem with her is a lack of contribution and personality. Katakuri, Perospero and Cracker had contributions to WCI's plot. In Perospero's case he was also a big character in Wano. Katakuri and Perospero both have engaging personalities. On the other hand, Smoothie is a blank state that does nothing.
Giving characters like Smoothie, Snack or Compote defined personalities, skillsets, and a role to fulfill is more important than outright replacing them. They are characters first and combatants second. For skillsets, they should be Haki-only users. Or Smoothie could use her Devil Fruit to complement her Haki/swordsmanship. And for roles, well, they should be more than just obstacles for the Straw Hats to overcome. Show their interests, competence, positions in the Big Mom Pirates, and their dynamics. The lack of these aspects is my biggest problem with post-timeskip antagonists bar Doflamingo. Most of them have just been brutish or arrogant obstacles.