Appearance of nipples after reduction
At 12 days post-op you are still very swollen. Since the nipples were left alone by the surgery and only the areola reduced, the nipples should, eventually, regain their former shape. But it may take several months for all the swelling to go away and the nipples to regain their shape. Good luck to you.
Breast reduction and nipples
It can take several weeks to months before the swelling and soft tissues relax to the point where your areolas are not longer on stretch leading to flattening of the nipples. Good luck.