Just like what FluffyCow said, before shes Harley Quinn, Shes Dr. Harleen Quinzel, she just chooses to live a life of excitement rather than be stuck in a boring office all day. lol.
Don't be such a tsundere Bruce, she makes your life a lot more fun and you know it :P Also, mom can't complain too much, Harley did make it out of college with a doctorate and Ivy is a real sweet girl when you get to know her (as long as you're not hurting nature) !