Symbolism, Imagery, Allegory
The fashionable ladies of Ratbridge symbolize that most unfortunate phenomenon we all know too well from the playground: peer pressure. They've bought into a special kind of groupthink, which has them mindlessly following the whims of Madame Froufrou instead of thinking for themselves.
When faced with the possibility of not gaining access to that week's hot trend, for instance, the ladies "started a desperate squeaking" (13.23). It's almost as though they're incapable of human reason. And Madame Froufrou (a.k.a. Snatcher in disguise) certainly makes it out to be a matter of life or death, telling the women that if they're not fashionably rich, "'you must cast yourself from this world of glamor and retire to your true miserable and rightfully low position'" (13.36). Way to raise the stakes, dude.
Because we are just talking about fashion, aren't we? Well, yes and no. It seems like to these ladies, fashion has come to represent social acceptance and popularity, and to be without either is apparently devastating.
The ladies never really stop drinking the Kool-Aid of fashion-as-peer-pressure, either. When a visiting Frenchwoman dispels the lies of Madame Froufrou, the ladies all rush to have their bizarrely-shaped butts shrunken back to normal. Do they question the need to fit in? Do any of them decide to keep their weird butts because they actually like them? Nope. When fashion is mere following, all we see is a zombie-like adherence to whatever the cult leader says is true.