Because The Joker is torturing Harley Quinn in the trailer. She's dressed in her psychiatrist uniform, so presumably this scene takes place before she joins him and becomes Harley Quinn in earnest. The Joker already has his tattoos in that scene.
And welcome aboard!
Uhhh...well...damn. There goes that theory! Oh well, I guess they're just bad tats then. It happens. I have two of them myself!