I really WISH and HOPE that Smoothie will become relevant at some point, but I wouldn't go as far as to say that she MUST.
Our fixation on Smoothie instead of any other sibling from the BMP is only because she's strong, nothing else. Well, but strength is important, right? Of course, it's a battle shonenโฆ Nonetheless, the importance of strength ultimately depends on the story alone, so that's why I think our expectations can be warped sometimes if we don't pay attention to context and intention.
The difference between someone like Rebecca and Smoothie is that Rebecca is a full blown character with a story about her (with disappointing development)โฆ while Smoothie is not really a character at all. She's only data that fills the gaps of worldbuilding.
You see, when we meet a big organization like the BMP we're supposed to be introduced to dozens of underlings and some of them should be really strong because it's a Yonkou crew, so that's why Smoothie must exist by the sheer demand of the worldbuilding. But that doesn't mean that the narrative should give priority to the strongest. That's only an assumption made up by arbitrary criteria, even though it makes sense in many circunstances.
If WCI were a traditional arc where the villains are defeated by the end, then it would be somewhat imperative to give focus to Smoothie since she would be paired against one of the protagonists. At a minimum she would get a fight like any usual subordinate, and at best she could also become a developed character like Katakuri fleshed out through the fight.
But WCI had this odd dynamic where the heroes were avoiding the antagonists during the entirety of the arc. The Strawhats were always escaping or hiding or quickly dealing with whoever was obstructing their way, and their survival depended on not being caught. And because of that Smoothie was in the awkward position of not having any role to play as she was too strong to be kicked around by the Strawhats like it happened to Oven and Daifuku in some scenes, and she couldn't catch them either. Unlucky for her. Aside from that, we can wonder if Smoothie couldn't have gotten at least one random cool scene, but how much would it actually make her better?
At last, I should point out that in regards to the personal drama being told in the arc, the main characters were Sanji, Pudding, Big Mom, Judge, Katakuri and Luffy. And that's a healthy number of protagonists. The rest was secondary or extra. And I honestly see no reason to add Smoothie to this cast just because she's a Sweet Commander.
As for her future, who knows what Oda might be preparing for Smoothie. It all depends on the future of the Big Mom Pirates, really. If the BMP become standard antagonists once again, then Smoothie will probably get her fight and some of the spotlight. But if the BMP get an alternative ending without fighting the Strawhats, then maybe we'll never really see Smoothie doing anything noteworthy.