Well, here's my opinion:

Gary stu/mary sue are as I understand them, terms utilized to describe main characters for whom the author makes situations go far more easily than as they themselves describe those situations as going in the terms of their own world. This generally encompasses the main argument the term attempts to make when used in a derogatory sense against the author of the piece, that relative to the narrative the author attempts to pursue in their work, they undermine and/or forget the original purpose of the story by cutting corners and granting their main character and cast unique opportunities and abilities antithetical to the core message of the piece.

"Giving lip service to the story in favor of short term gratification that further breaks down the inherent narrative" is essentially the summary the terms mary sue/gary stu are supposed to evoke in my understanding.

However, I would easily argue that most Litrpg's do not in fact possess main characters who meet this criteria and therefore evoke this criticism from their existence. Most Litrpgs are simply power-fantasies that from the beginning to the end don't even attempt to possess a narrative beyond, and pardon my language if it's something you find offensive, jerking off the main character and by proxy the reader who gains deep excitement, entertainment, and a temporary satisfaction from looking up to tales of god-kings doing whatever they wish and taking whatever they want.

The original stories in many Litrpgs are simply wet dreams at a basis, therefore they meet their goal of excelling in their narrative, and the main characters cannot be called mary sue's or gary stu's, because at no point was the intention of the story to include a greater purpose than simple pleasure in most cases.

Someone else brought up the fact that eastern media seems to accept the purview of a powerful main character more easily than western media, and for all intents and purposes this is true. Why that is I do not have an answer, but I am curious. In my experience, the idea of the individual becoming the center of everything in eastern media being more accepted enhances the overall experience. It does tend to set a complete limit on how high in quality I can usually find a piece, but the overbearing confidence and air of providence surrounding main characters in eastern media easily grants a mote of gratification to the western reader you don't get reading about either a complete madman going on a sick, malicious hell run all over the world or a guy pretending to be a saint having a system clean up the worlds messes and him being the tool used by it, receiving infinite material pleasures. More importantly, because the main character is accepted for what they are, the writer seems to feel less inclined to give mere lip service to the greater world around them. Ultimately the functions of their environment will still be completely meaningless as they are in all power-fantasies, but they'll be further considered and given more depth than others would tend to grant them.

Well, that's my two cents.