Yeah; I think Mary Sue is a very useful term for a concept that really didn't have a word before (notice how it's spread far beyond its fandom origins -- a surefire sign that it fills a needed linguistic niche) but along the way it seems to have lost the specific set of connotations that it started out with; I don't even know if a lot of people using it now have any idea what it used to mean. I'm not really convinced that it's wrong so much as a case of linguistic drift, except that the word's become so generic that, as you pointed out, it's in the process of becoming just another word for OFC (or, rather, "OFC that the reader doesn't like").