Isn't it possible that the assignment she first got was a better post on a "lesser" ship and thus offered better opportunities for advancement? Maybe cadets should have to work their way up a bit no matter who or how good they are?
Define 'possible'. Do you mean that, in-story, Uhura cannot know for certain that the post she perceives as inferior is not necessary or better for her? I have nothing that can prove she could. OTOH if you're just inviting me to speculate that that may in fact
have been the case, then I'd have to disagree. The Enterprise is the flagship. Nothing really trumps that.
But none of that matters. 'Destiny' is as close as we can get to an overriding theme of ST09, and it's Uhura's destiny to be, for a time at least, the Communications Officer of the USS Enterprise, Captain James T. Kirk commanding. If you don't like the attitude she takes in the course of fulfilling that destiny, what can I say? You don't like it. I can't really talk you out of that.
Trek is often guilty of giving 'inappropriate' tasks to main characters