Why does the lead singer of flyleaf have a tattoo if she's christian?

Getting tattoos is not anti-christian necessarily. The only reference The Bible makes to that is not to harm yourself in anyway, not everyone sees getting a tattoo as self-harm. These days many normal daily activities might be considered self-harm, such as drinking caffeine, eating fatty foods, taking over the counter medication, taking prescription medication, et c., so whether or not tattoos are self-harm is a subjective thing.