Daisy Ridley herself has come out and stated that she believes the ‘Mary Sue’ tag is sexist and inappropriate.
http://comicbook.com/starwars/2017/12/21/star-wars-the-last-jedi-daisy-ridley-mary-sue-rey/
It’s become a meme, and I can see her point, but I can also see the term being used to silence critics.
“So, you didn’t like Rey’s character development? Are you implying Rey is a Mary Sue? You must hate women in lead roles, you misogynist pig!”