Yeah, Arkham City's Nightwing is a dude, but he's a skinny dude—especially compared to the hulking giant that is Batman—so it's totally within reason for a woman to cosplay as the former Boy Wonder.
Am I the only one who thinks cosplaying is dumb if you aren't trying to look like the actual character? The whole point of cosplay is to do just that.
In this case, yeah the costumes kinda cool and she's sexy and what not, but Nightwing is not a woman. Link is not a woman. Zelda is not a man. Samus is not a man.