I've read one too many articles that have dismissed her with one-word descriptions such as the following: "frumpy", "ambitious"-- turning the focus to more overtly feminine counterparts, Betty and Joan respectively. While I adore both Betty and Joan, I feel that critics tend to tragically overlook Peggy-- I've almost come to find Jezebel's visible preference of seductress Joan over the blatantly more progressive Peggy offensive. Yeah, so sometimes she's a bitch-- she's human and every character on such a realistic show needs some weak points. But I can't do anything but admire her for skill, climbing the corporate ladder through creativity, not dirty tricks (which could be said, maybe half-heartedly, of her mysterious male double, Don). If she wasn't such a required focal point for the show, the pilot would not have revolved so much around her first day at Sterling Cooper. An outcast in both stages of her work environment, a great article I read on her suggested that her character's weight gain at the end of season one seemed to be a strong statement against the gender roles of the era-- the constant unspoken requirement of being pretty. Sudden physical anomalies aside, Peggy is promoted by her notably sexist male co-workers. These are just a few of the many reasons that I find her to be such an important, resonating character. Without her, I firmly believe the show could not go on.
So yeah, she may not be a Jackie or a Marilyn, but is that really such a hindrance? After all, I don't think anyone would want to be one of a hundred colors in a box, would they?