Aye a role is pretty vital at this point since all of the crew have one. All of which have been pretty vital to their characters.
Carrot as lookout is important as her mentor Pedro was experienced with it. We even see him jumping up to a high place and watching out for the dukes exactly like Carrot does. Not to mention the whole thing with the dawn of the world thing that the Straw Hats will do which Pedro wanted Carrot to witness.
It's also a role we've seen constantly throughout the series, even on the Sunny. Luffy even has a desired personality for a lookout which Carrot fits perfectly (when he said Robin wasn't excited enough to see an island).
It's a pretty important hurdle to jump over that a lot of recent nakama canidates have been lacking and fellas have to stretch to figure out a potential role for them.
Even Vivi you can comfortably say can be a diplomat role or whatever the pirate equivilant is cause pirates strike deals all the time. And Vivi showcases how important it is on Drum Island when the locals are blocking them from entering.