I don't think most are getting ideas about fashion. I think, most often, they're either thinking something like, "Oh - I like her outfit" or "Oh, I like that sweater she's wearing." Or else, they're thinking things like, "Why on Earth would she wear something like that?" They may even be thinking something like, "She looks familiar. Is she someone I went to school with?"
I don't think this is just a girl/woman thing. Some people notice other people more than others do. I'm not someone who pays much attention to what others look like (and I'm a woman); so if I do, I'm either thinking the other woman is either awfully attractive and I'm admiring her, or else I'm thinking she's wearing something that doesn't look good on her. I'm never comparing her to me, though. It's probably no different than if I see a particularly pretty, ugly, or neutral building that, for some reason, catches my attention. I'm me. Other people (and things like buildings that catch my eye for one reason or another) are what they are and have nothing to do with me.