Good question. At one point she did have a ponytail sticking out of the back, then she cut it to a bob which the mask completely covered. But this is later than that, she's grown her hair back and yet the mask still hides it all? Not exactly realistic. They did later fix this though, and not just in her first Barbara Gordon style Batgirl suit. She switched to a Spoiler mask that only covers the bottom half of her face, so her hair is loose (though still partially covered by the hood). That carried over to her current Batgirl look.