Let’s look at what kinds of Mary Sues people will point to. People will claim a female character is a Mary Sue if she is a love interest. Put a female character within a foot of a male character, and people will scream “Mary Sue!” Why does someone falling in love with her make her a Mary Sue? Well, she hasn’t “earned” this awesome dude character’s love. What has she done to show she’s worthy of him? Fans miss the irony that this line of logic makes the male character seem more like the Sue in Question, as he’s apparently so perfect one has work for his coveted love and praise.
The idea that woman has to “earn” any power, praise, love, or plot prominence is central to Mary Sue. Men do not have to do this, they are naturally assumed to be powerful, central and loveable. That’s why it’s the first thing thrown at a female character- what has she done to be given the same consideration as a male character? Why is she suddenly usurping a male role? “Mary Sue” is the easiest way to dismiss a character. It sounds bad to say “I don’t like this female character. I don’t like that this woman is powerful. I don’t like it when the plot focuses on her. I don’t like that a character I like has affections for her.” But “Mary Sue” is a way to say these things without really saying them. It gives you legitimacy.
If a character is badly written, there’s generally something much more problematic than idealization going on. The plot will be dull and the character will perpetuate harmful stereotypes while other characters act oddly. For instance, Bella Swan is one of the only characters I’d even begin to classify as a Mary Sue, yet it’s not really her supposed Mary Sue traits that bother me. I don’t mind that she gets what she wants and everyone loves her, that she’s Meyer’s power fantasy. What I actually mind is that Stephenie Meyer has her perpetuate harmful anti-woman stereotypes- women need to be protected, women are shallow, women’s worth rests in desirability. That’s what’s actually harmful about her and worth discussing. I would criticize that rather than even get to the fact Bella got to be “too perfect and powerful”- that’s just a tiny, insignificant thing not worth mentioning in a huge pile of problems.”