Somehow both yes and no. Mary Sue in that she starts out basically perfect and ready to fight, a stupid moron because she made terrible choices based on ignorance and what she WISHED was real instead of what was actually real.
Somehow she's the only avatar to fuck up the avatar state and lose access to the entire history of avatars as advisors. Oh and then she punched a permanent hole into the spirit realm, because we all know how nice and willing to get along spirits are with people. And that was how the series ends. Apparently punching a hole into the spirit dimension is supposed to be uplifting or inspiring to the audience as a way to close out the series.