In the play "Truth Values" the protagonist ends up dressing "inappropriately" in math grad school as a way of... mmm... acting out? Showing independence? Differentiating herself from others?
My husband once went to an AMS regional meeting and pronounced it a markedly unattractive group of people.
In graduate school I generally wore jeans and t-shirts and sweatshirts. Don't want to stand out.
In undergrad that really was driven home for me, as when you wear a dress at an engineering school all the guys ask, "Hey, is it laundry day?"
My current department seems to have a different code. There are a few more people who had a corporate life, even if only briefly, and so there are blazers now and then and business casual pants and the occasional real suit or even a woman who looks like an Ann Taylor ad. No, I'm not monetizing the blog yet... although their triacetate pants really do last forever (I have a pair that is going on ten years old).
MAA conference attendees are less gray than AMS conference attendees: more likely to wear colors or even ruffles. Maybe it's because there are more women and the women don't feel this American pressure to wear drab colors and a backpack.
In Portugal and France mathematicians are orders of magnitude more fashionable than in the US. Why is that?
I do try to dress like a professor is "supposed to" look in some freshman's warped view of reality, at least at the beginning of the semester. I am small and female and relatively young-looking at this point. Haha. A student in the library was astounded just a few weeks ago that I got the long check-out period for books -- she said she thought I was a student. This means I can't wear that grad-school uniform of jeans, holey sweatshirt, and Chacos. Looking poorer and scruffier than my students... mmm, just can't do it. But it's rather an experiment to figure out what looks adult without looking matronly or frumpy. Why does it matter anyway? There are plenty of normal fashionable things I don't do because it seems like an absolute waste of time (shaping my eyebrows?). Why do I even care if I look matronly? Maybe if I was more immersed in the mathematical tribe rather than living among the art historians and French professors of a SLAC I wouldn't notice so much.