One could make the argument that she's just that arrogant and confident about her skill that she did not feel the need to put on armour, against the protagonist that she's clearly belittling before the fight.
Personally though, I would've appreciated it if she actually wore the armour. In this state, at best she looks more like a swordswoman than a knight. At worst, without the cape she looks more like a courtesan with such a revealing outfit.