I think to see Mary Sues as a genre could be really helpful. I can like Mary Sue stories, but many I don't. I think what makes me dislike some of them is not so much how many awesome qualities she has, or how much it is self-insertion/wishfulfillment, but whether the characters I like end up like idiots to warp the story around the Mary Sue or not. I mean, I have read a bunch of Mary Sue stories that were intentional, and met plenty of the checkboxes (competent with tragic past, romantic relationship with the hero etc), and yet not parody. They weren't the pinnacle of craft but competently written. And they were fun. I think the ones I enjoyed most were of the type where our regular world with an average yet awesome self-insertion character meets with the heroes and then stuff happens. Like time-space plot device strands a Jedi in present day earth where he meets our heroine and they try to get the Jedi back were he belongs, and the displacement makes the ordinary person know a bunch of relevant stuff in a believable fashion that the Jedi doesn't etc.