OK lemme tell you something.
Nia isn't a Mary Sue for the simple purpose that she doesn't EVER break the actual curb of the show.
Simon is an insanely powerful mecha pilot who breaks reality with a punch.
Kamina is a douche with glasses and a powerful voice who convinces thousands of people to join him on some ridiculous journey because HIS FATHER IS DEAAAAAAAAAAD.
By comparison Nia is a princess who manages to win Simon's affection via being soft yet supportive. How is that anymore Sueish than ANY OTHER CHARACTER in TTGL?
Face is TTGL is basically Sue/Stu central if we go by powerlevels alone.