Mary Sue (which can also be "Marty Stu" in the case of male characters) is a character who is an author self-insert. He/she is essentially the author projecting their wildest fantasies of living in a particular universe onto one character. By that definition, Michael is very much a Mary Sue. The non-stop ego stroking and validation she receives (from Georgiou, Sarek, and Saru) is just one aspect of her Mary Sueishness. There's also the fact that she is the driving force in nearly the entire plot. SHE demands to fire on the Klingons - naturally her life experience is more valuable than her captain's 30 years of command, right? When she doesn't get her way, SHE seizes command of the ship. And of course her point of view turns out to be correct, how could she be wrong? SHE demands to pilot the worker bee. SHE insists on capturing T'Kuvma instead of killing him. The Mary Sue character must be the center of the universe at all times, because again, she is the personification of a writer's fantasy of serving on a Federation starship. Being a lower-ranking officer is boring, so naturally the Mary Sue character is smarter than her superiors and always knows the correct course of action, and other characters yield to her superior judgement. Then, at the end of the episode, she faces the "consequences" of her actions - which consist of losing her friend and captain, whose meaningless death acts as a motivation for Mary Sue to continue her profound quest to Find Herself and discover her individuality. And then she is imprisoned until the start of the next episode when her Superior Intelligence is needed yet again for another mission.