This is actually lampshaded in the Delta Recruitment event: while running the Tutorial, you encounter your future self, who has reached the highest ranks in the game and told of what's going to happen with the Iconians.
Even giving permanent command to a cadet who hadn't even finished their senior midshipman cruise is quite frankly ridiculous, even with the handwave in the backstory that Admiral Quinn had started a policy of fast-tracking the promotions of promising junior officers.
Artistic License Military/ Overranked Soldier: Regardless of the player's competence, they're still less than two years out of the Academy and yet a full fleet admiral by the end of the Delta Rising story.
Beyond that, the diplomatic commendation system allows you (due to how it works, it's liable to happen sooner or later) to become an ambassador, complete with title and special uniform.
Ambadassador: You are a Starfleet officer, and that means you are supposed to be this in the absence of an actual diplomat.
Because everything from his/her/their appearance to species can be wildly different from player to player, only general characteristics will be listed here.