Can people fucking learn what a Mary Sue is already. Christ.
A Mary Sue, simply enough, ruins story by waving away obstacles with contrived bullshit.
Stories work on the premise that characters go along the path of storyline, something blocks their way and the character must overcome this. It could be simple shit like, finding their keys or something more extreme like defeating a big monster. Either way all stories can be brought down to this simple concept.
Mary Sues ruin this by avoiding those problems outright. Rather than finding interesting ways of overcoming them they just pick some shit out of thin our like "oh my great great great uncle just happens to have been an amazing mage, I inherited his power" And instantly defeats the monster. They've done nothing to obtain this, it's never caused them no issues have either, they can just do it and sort an issue out without any form of interesting reason as to how.
Simply put. Mary Sues make stories boring by missing the entire point of story telling.
They are not simply over powered.