I don’t like the ST myself, but the Mary Sue argument is very silly and indicative of one’s attitude towards women. “We don’t hate women, we hate poorly written characters” doesn’t apply when a person defends the prequel trilogy over these films.
Your right the prequals were just poorly written stories and films where as Force Awakens and Last Jedi whether you like them or not have more depth within the narrotive. Personally I like TFA & TLJ and overall can overlook their faults to enjoy the story they tell and look forward the future ones to come when the Skywalker saga finishes.