What's mary sue about her? We see her, and other characters in the show, develop their martial prowess through rigorous training
Plus, because she's royalty, she has the benefit of special tutors and the personal ambition to get the most out of their training.
Her flaw is, iirc, her arrogance; there's times when she's about to get what she wants but it gets away from her because she plays with her food.
Everything is so "easy" for her that she's constantly underestimating her opponents; I mean in the end she gets beaten by a length of chain and some water