Yes, the boob window, as it's known. One writer once tried to explain it by saying she left a hole because she didn't have a symbol to fight under like the rest of the superfam. I just see it as a product of its time.
What I like about Peej though is she's often drawn with more muscle than your average female superhero, which is nice to see. She's also a pretty fun character. I like when Amanda Conner draws her, because she does such great facial expressions.