I think the point that makes this question relevant to Lower Decks is specifically around Beckett Mariner and whether she fits the Mary Sue trope. I've seen this accusation thrown around in angry-man YouTube videos, the type that if you watch them long enough, it's clear they're part of the so-called "Anti-SJW" crowd that will never be happy as long as the main hero roles are anyone other than white men. They won't outright admit that, but that's the total vibe on all those videos and it's gross.
The idea that Mariner is a Mary Sue is easily disproven. She's not, in my opinion, badly written. She's not all-knowing and all-perfect, in fact she has some rather obvious character flaws that I'm sure will be developed as part of her arc. She is, I think, over-confident, slightly arrogant, and disregards Starfleet authority just a smidge too much (again, in my opinion). I think this is best reflected in the microcosm of her outright stating to Boimler basically that she is "always right, never wrong, you should do what I say, got it." She might believe that's true of herself, but that doesn't mean it is, or that the writers believe that it is. I mean let's be real here, she drinks on the job. This will need to be addressed at some point.
Now, sure her approach has worked out well for her these first two episodes, in contrast with Boimler's by-the-books approach proving less effective in real-world situations. But there could come a point soon where the consequences of her actions might catch up with her; We are seeing more of Boimler's arc first, I think, him learning from her, and her growth may come a bit later. We've learned she's even been demoted and shuffled around to different assignments as a result of her difficulties, that's for sure going to be addressed more soon.
She is, however, indisputably more experienced as an officer, and has better street smarts than Brad Boimler, neither of which are superhuman traits and achievements on her part. Boimler's not bumblingly, unbelievably stupid, or unrealistically failure-prone. He's just kind of average, compared to the Starfleet heroes to whom he aspires, and she just (despite her character flaws) happens to have been around the block a few more times, and has experience he lacks. It's totally believable.
But I think that's what actually really sets off these YouTube guys. Here you have a white dude character who is *not* perfect, who is obviously flawed in a way that's occasionally played as goofy for laughs, and here you put him in a workplace where a black woman is successful next to him, and the storytelling conveys that he's got an opportunity and an obligation to learn from her.
I think that really grinds their gears, and I think it's really obvious why that is. They are judging these characters by unequal metrics. Over-perceiving unflattering imperfections imposed on Boimler's character as unrealistic and unfair, and again over-perceiving the positive traits associated with Mariner as somehow inappropriate or undeserved. Too bad these guys can't actually view the show through the lense of Roddenberry's vision which they claim they hold so dear!
There are things about both of the characters that I don't like. Boimler pulling the phaser on Mariner and the farmers. Casually mentioning Section 31. Boimler disregarding Mariner's experience. Mariner not reporting the energy creature, and getting drunk with K'orin on the shuttle. But characterizing their interactions so far as that she's supposedly a perfect do-no-wrong figure, and Boimler the perpetual inferior in all respects, not fair or accurate. It's a bad read, in my opinion a sexist and racist read.
By comparison, I think Discovery is bad for other reasons, and does suffer from bad writing. But I hold that criticisms of Michael Burnham as a Mary Sue are unfair and inaccurate for similar reasons to Mariner in Lower Decks (Burnham is by no means unflawed), and for various other reasons already articulated by others in this thread (ie. by comparison with prior Trek series leads).
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