I don't think you're wrong about the film's inherent misogyny, but I did think that Marla was a likable character. She was smart and, especially towards the end of the film, a welcome respite from the increasing derangement of project mayhem.

You could argue that this is a case of Bonham-Carter's performance outdoing the script, and that may very well be the case. Alternatively, it's possible to point out that, as the narrator's grip on sanity slackens, so does Marla's anti-social attitude and transgressive behavior, and that she does come to represent a sort of restrictive femininity. By that point, however, she's clearly right, and probably the only fully sane character in the bunch.