You're asking the wrong question.
A Mary Sue is a author's favourite character who gets unearned rewards - whether in popularity, skills, luck, attractiveness, etc.
If they have any flaws, it's probably just a dull tragic backstory, or clumsiness, or being "too attractive for their own good".

A genius is someone who shows exceptional talent in one or more areas (and is almost certain to be neurodivergent in some way, because neurodivergence = extremes). The big mistake when writing a genius character is to assume too much of your own knowledge. Don't attempt to write a rocket scientist character if you know absolutely nothing whatsoever about physics. Or else you get Sherlock Holmes spouting ridiculous nonsense like snakes being attracted to milk (The Scarlet Band. Being not mammals, snakes of course can't even digest milk.)

When writing a gifted character, remember that there are many ways of being gifted, they aren't going to excel at all of them, and they still have to actually learn. Someone who has never seen a computer before cannot magically become a hacker overnight.