True! If she's a Starfleet brat and got moved from ship to ship between mom and dad her whole life, she's most certainly had more on-the-job training than most top-of-the-class cadets. Most of that world would have been completely second nature to her, without the responsibility until she went into the Academy herself, which I'm sure was fully expected by everyone concerned.
Reminds me of Wesley's rise and fall in TNG, with his eventual disillusionment and cynicism jading his desire to continue on with what everyone believed would be a good career for him. Mariner was probably similar, to a degree, at least with respect to initial interest in the Fleet, only to have the idealistic notions of Glory, Duty, Honor, etc., quickly dashed from one bad mission.
And they wanna talk about a Mary Sue - Wesley was it! Constantly figuring out the problem of the week while the rest of the senior officers were either walking into bulkheads or drooling in cups.
I REALLY want to see Mariner's back-story leading up to the Cerritos. Must have some flashbacks soon!