Margot is indeed an icon. I've seen many a fashionista hold up a vintage fur coat and exclaim how they'd love to wear it and "feel just like Margot Tenenbaum," and her name comes up in the description of virtually every vintage striped polo dress listed on Etsy.
For a character who dresses almost like an animation (in the same thing all the time), she sure has been influential and inspiration to fashion lovers everywhere; you don't see people scouring vintage shops looking for a dress just like Lisa Simpson's, but everyone wants to look like Margot.
Her basic look hasn't changed since she was a kid: straight, all-one-length, blonde hair with a barrette at the side, and dark eyeliner. It's so wrong that it's right.
And her outfit hasn't changed either: blue, striped polo dress; and a huge, brown fur coat. And don't think we didn't notice that Birkin, Margot.
In fact, the only time we see her in anything but her signature dress and coat, it's a nude colored slip that hugs her slim body and makes her look both washed out and fabulous.