One theory I have is that there are two personalities to Mona: the 'A' side and the softer side, kind of like Dr. Jekyll and Mr. Hyde. Do you think there could be something behind the duality?
I often think, in art and life, we respond to those who personify our own darker instincts. Maybe that's why these four girls were so devoted to Alison. I certainly think that the reason Jekyll and Hyde is such a powerful, enduring story is that we all have shadow sides, and we enjoy stories that ply out those shadow stories for us so that we don't have to experience them in life. The girls are hiding something in any given episode. We, the audience, enjoy seeing them lie again and again, though I doubt any of us want to emulate their behavior and experience the consequences. And Mona is just a larger exaggeration of that impulse to indulge the shadow.