Mariner is just an unorthodox officer, trying to fit into an extremely orthodox organization.
She's just Ms. Perfect and Awesome and Right About Everything! Never wrong! Never makes a mistake! I mean, she's just the bestest!
That's thin and grating.
So you think Boimler would’ve been fine on that mission, handling a Klingon on his own?
Someone thought he was able to his. His job was to fly him to embassy/conference/whatever and they come back. Mariner barges herself in, gets herself put over PH on the mission then goes and gets the Klingon drunk which spirals into the string of problems which do come from PH's inexperience.
Which, yeah, I'm sure that was her plan the entire time, to get the Klingon drunk, have him steal the shuttle, then have her and PH walk through this area, where he'll make a series of mistakes she (The Bestest!) gets to correct and constantly show him how wrong and inexperienced he is while along the way showing and saying how cool, awesome, and best she is!
I don't find that to be a good, or interesting, story or character.
Jadzia has subtly and was wrong on occasion and didn't constantly shout her lines and act like she was the center and focus of the entire room, and her knowledge and experiences came from having lived through several lifetimes. Here in this episode PH says he and Mariner are "like the same age" but, yet, she's got all of this vast and awesome experience that makes her cool and great and knowledgeable at everything. She's flawless and the best! Just love her because there's nothing about her to not love!
She's the best!