I dont think that a character being "Mary sue-ish" is an inherently bad thing
I often hear this word used for characters in a negative sense. I am not knowlegable enough to say whether a character is "actually" a Mary Sue, I only talk about characters that many people criticise for being like that.
However, I dont find an issue with this, not inherently. I enjoy some books with these types of characters, on occasions, and depeding on the book. And since many of these books are very popular, a lot of other people like them also.
I see a lot of people who like these books push back to point out how the character is "not actually mary sue". And there are good points on both sides. But in my mind I just think "who cares". Lets say they are. So what, that does not make the books bad, if you enjoy them, they are good to you.