Yes, the cover looks horrible but Plath wrote poems at that time where she described herself as a mannequin. She did that to criticise consumerism, expecially towards women (as she experienced it, at least). So this might be a reference to these last poems she wrote. A bit like: I'm just a doll, shut up now, leave me alone, I do what you want.
I like the
cover of the german version a lot, it illustrates the experience Plath describes in this novel really good, in my opinion.
Sorry for beeing off-topic