This was originally going to be a comment, but it got too long so I decided to make it a blog post.
A lot of people seem to have a misconception of what a “Mary Sue” is. Just because someone is beautiful doesn’t make them an automatic Sue.
It's when those characters get way too much attention as a constant, irritating reminder of their awesomeness.
A Sue’s awesomeness is constantly shoved in your face, and they get WAY too much attention
See, at least cats like Feathertail and Cinderpelt don't make constant appearances and annoy the @$#! out of readers. The main issue that makes Spottedleaf a Mary Sue, in my opinion, is that she gets way too much attention. The characters in the series obsess about her, which detracts the enjoyment value of the story (don't think I'm the only one who thinks this). She appears way too much in dreams, to the point where it's obvious that the Erins are spoiling her with so much attention and shoving the fact that she's so "pretty, wise etc" in the reader's face. Not to mention that she has to be "unusual" and her appearance description is 2348923749823 words long. It’s annoying.
And she does this even though the original series is over (and she's still mentioned as pretty if I'm not mistaken, haven't read it in a while)! She appears in almost every Goddamn book of the New Prophecy Series, even though she's dead. And she just has to make a cameo in books where she's long gone and basically irrelevant to the plot, like Bramblestar's Storm and Ravenpaw's Farewell.
Everyone loves her. Those who don’t? You're an evil b#!@!
It's evident that it's not just Firestar who thinks she's pretty. Every character thinks so. Even Tigerstar acts all nice around Spottedleaf, which is more evidence to point that she's a Mary Sue because even grouchy characters are made softies around the pretty young perfect Spottedleaf who can pacify just about anyone! (At least she hasn't redeemed a villain, that would be worse) And of course, long after she's dead, there's a new novella about another one of her love interests (as if we didn't have enough of her). Thistleclaw is willing to fall head over heels so quickly for an apprentice, which says all things about the attention she attracts. This also contradicts the claim that she's only pretty in Firestar's opinion.
Most cats think of her highly even though they don't have grounds to do so. As somebody mentioned in a blog comment, they think of her even more highly than Bluestar, who's a leader. And Spottedleaf has no grounds to be more highly respected, TBH (she's a young inexperienced medicine cat...). Whoever disagrees with her/doesn’t appreciate her perfection/harmed her is through and through evil and suffers a horrible fate (see Clawface and Mapleshade, and yes this is a typical trait of a Mary Sue). Nobody "good" disagrees with her, if you never noticed.
(Oh, and have I mentioned that Firestar only met her for a few days? )
A Sue has no real, meaningful flaws
She has no flaws. Ok, some claim that her flaws are being a "stalker" and a "creep" but that's because we as the readers think she is. Do the characters in the series ever call her out for that? Does she ever get consequences for such behavior? Not as far as I know. This so-called "flaw" doesn't detract from her apparent awesomeness nor hinders her in anyway, in the author's lens. In fact, the characters are glad that she keeps appearing in their dreams! That's not a real flaw, because in the author's lens it's a good thing for all characters.
Our annoyance towards her is justified.