From Batman News:

Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn) is a title that seemed too long to be real. Fans thought Margot Robbie was just messing around when she scribbled it on the cover of the script. Robbie was deadly serious but only because the movie is not.

“It’s not a very serious movie, so we thought the title should reflect that,” Robbie said on Monday’s The Tonight Show Starring Jimmy Fallon. “Birds of Prey makes it sound very serious and then that’s kind of like Harley adding her ‘Hey, don’t worry. I’m in this, too.'”