Is White Lily Cookie a Mary Sue?

I've heard some people calling her a "Mary Sue". Can someone explain it to me?

However, here's my take on her; I like her, but at the same time I dislike her. I have mixed feelings for her. Something about White Lily Cookie made me irritated just a tiny bit, but I don't know what it is.

I shipped White Lily Cookie and Pure Vanilla Cookie just a little, but don't see the reason for people shipping her and Elder Faerie (I won't judge tho, I'll leave them be).

So, uh again, why is she called a Mary Sue?