It really depends on whether you define a mary sue as "a perfect, invincible character" or one that warps the story. I prefer the latter, and on Nohr side, that's definitely true. It's difficult to get into without spoilers, but the narrative stretches itself to absurd lengths to keep Kamui from appearing morally wrong in any way, shape, or form. Since Kamui is supposed to be the player, he can't do anything too... Well, he can't do anything the player would be unwilling to do without breaking immersion. The problem stems from this intersect, where Kamui has to do something to move forward the plot that the player may not want to. Therefore, the narrative bends to accommodate Kamui's (and the player's) reluctance.
Edit: Er, spoiler tag isn't working for me, so I removed the list.