Her cleavage acts as a heat exchanger for the armor. You could still have a covering over them that's a control surface only opening up when venting heat. That would look little steampunk as seeing venting of water vapor.
REAL TALK: Being sexy is internal and it's not the same as being sexual. It's how you carry yourself. Revealing clothing can be sexy. That doesn't make one, sexy. If you build elegance into your women it won't matter how little or how much you show.
Just ask a woman, not a girl, and she'll tell you I'm right ;)