A Mary Sue doesn’t cease to be a Mary Sue just 'cause she has ubermensch genes.
In this case it does. Rey doesn’t want her immense powers and in TROS appears at odds with his heritage.
And for people who say “character from nowhere is a new and bold idea”…
We’ve seen a hero from nowhere in the PT.
We’ve seen a hero from Jedi heritage in the OT.
Now we’ve seen a hero from Sith heritage in the ST.