Taking a popular franchise like Star Wars or Doctor Who and making it boring by turning it into progressive propaganda device. Typically it features unrealistically competent "minority" groups like female lead of Force Awakens or Last Jedi, the character being so characteristic, simplistic and cartoonish that it generates its own popular names, like "Mary Sue" in this case.
Mary Sue got her Jedi training by Luke completed in one day. Man, what a meh-gutting!