Most of the jobs in the crew are only ancillary to the characters, imo. But for the sake of the discussion, if the crew were to get a dedicated lookout (Carrot or not), it's worth considering what exactly Oda might want from this role.
In real life, the lookout is an important job but kinda simple and monotonous too. It's someone that stays put in a strategic position to be looking at the sea while the crew do their respective jobs or rest. It's mostly something you do in the open sea while nothing is happening, just watching the waves to see if something comes up, taking precaution against physical obstacles like reefs and icebergs, finding ships in the distance, shouting out about islands in sight.
That's why Usopp is not the Strawhats' lookout, as he was never the guy responsible to sit in the crow's nest to be watching the horizon monitoring the sea (instead, that has been a collective duty in the ship). That's a very different circunstance compared to a scene where the Strawhats are invading Onigashima and everybody is paying attention to the island, and so Usopp finds his foes and snipes them. Could a lookout have found them instead and warned Usopp? Maybe. Is it necessary? No, because Usopp should be able to do it by himself.
The question is if Oda could want anything more from a lookout beyond its basic description that I provided. That's a valid possibility, since in a universe with superpowers we could have an overpowered character for the role of lookout. The best example is Viola, who is often mentioned here in this thread, and it's true that Viola would be a fantastic lookout much better than Carrot could. She would open a completely new dimension to what the lookout could do for the crew, in her case almost to the point that it could become inconvenient for storytelling (just like CoO is frequently ignored by Oda). Nonetheless, we can all agree that Viola is not really a contender for crewmate, so there's no need to be debating about her specifically…
In any case, it's possible that someone else with a little less overpowered ability could fill the role of lookout and still be "the best in the world" for some other reason. Better than Carrot or any other current crewmate for the job. Who knows.
While I'm open to this possibility, right now I don't think that's what we're supposed to get. The hypothetical lookout could just be... a stardard lookout. In fact, the only scene in OP where we saw the Strawhats debating about what they want from a lookout, Luffy and Usopp were complaining to Robin that a lookout should be excited about adventure, that a lookout should scream with passion whenever he/she saw an island in sight. And, well, that's precisely who Carrot is, actually.
So we have three options: (a) no lookout, (b) an overpowered lookout, (c) a standard lookout who is apt for the job. All three are equally valid options, in my opinion.
If Carrot joined the crew, I'd say that she's closer to "c" than "a" or "b", but I don't see that as a detriment to her, especially considering the description of the job. I repeat what I said earlier that most jobs in the crew are only ancillary to their character, not the reason why they're in the main cast. If Carrot joined the crew, it wouldn't be because she is a fantastic lookout, instead she would join for her own narrative purposes and just be a fairly good lookout… or maybe don't even be a lookout at all.