The whole point of the term "Mary Sue" is to be able to identify a bad, yet often used character trope in storytelling.
Yes, the original definition of the term was a character being too perfect. It came from people's feelings about self-inserts in fan fics. However, many eventually realized that perfect characters don't always pose the problems Mary Sue's do in stories, and sometimes even flawed characters can bring about those same problems.
Thus I, and others, now argue that a Mary Sue is defined more by how they affect a story than how perfect they are. A character being perfect is indeed often a symptom of them being a Mary Sue, but that aspect shouldn't define what one is, especially since paragons are a thing.
When the plot and themes of a story bend around a character to the point that it is clear the writer(s) favor them, THAT is a Mary Sue, and said characters are problems because they make stories unbelievable which often also make them unenjoyable.
Note, this is not the same as the writing in a story simply supporting a character. As long as the writing maintains a secure about of believability, it's fine.
I’m so confused. Isn’t a Mary sue a character who has no flaws and is always perfect and never struggles?