The vapid expression is because she's "sexy but nor sexual."

That might not be an exact quote, but it's the gist of it. The idea being that she's hot but she has no desire for things that are hot herself. Her job is not to see, but to be seen. Which is really creepy, but fits accounts of the infamous sex tapes - in which she is bored enough to start talking on the phone, while, um.....

And also (sadly) makes a sick sort of sense if she was actually used as a model for some female comic book characters.