I feel for you, Natalie/GP. I remember how weird and awkward it was to try to make friends with the other parents when my daughter started kindergarten, even though it was a small school that went out of its way to be a real community for the families. I loved looking around and seeing parents (that school was great at getting dads equally involved, so it wasn't just moms) in full goth regalia and colorful hair, in colorful robes and hijabs, in hippie/boho outfits, in crisp business suits, in cycling gear, in Birkenstocks, in high heels, in kippahs, in sleek preppy bobs, in waist-length dreadlocks. There was no worry about blending in or not, because the parents were all so different from each other. I thought surely I could find somebody in such a diverse group to be friends with, but it wasn't that easy at first. DD started 4th grade at a new school this past year, and I have had to go through the process of fitting in with the other parents all over again.
I still remember the parent coffee hour on her first day of Kindergarten and feeling intimidated by the ones who looked more sophisticated than I. I've been a university administrator for most of my career, in settings that are very casual. The moms in more formal business wear all looked so much more dressed up than I ever have to be for my work. I remember one mom who looked so fabulous that I felt she would never have any interest in being friends with me.
If I had found myself in such a conversation with her early on, I could totally have seen myself blurting out something as thoughtless as what that woman said to you, perhaps in an attempt at self-deprecating humor. Then I'd spend the rest of the school year kicking myself for having said such an idiotic thing, and being too embarrassed to ever look her in the eye again! Which of course, would only serve to confirm the initial impression of rudeness.
So, basically I'm saying forgive Ms. UnSnazzy for that one thoughtless sentence, assume positive intentions, and don't carry a grudge against her. Keep being your stylish self and being open and friendly to everyone. She may eventually realize that you don't look down on others for wearing workout clothes or for being less stylish. You may find that she's really a very nice person when she's not feeling nervous or insecure. Or you may end up never getting along with that particular woman, but you will make friends with other people. These things have a way of working themselves out with time and familiarity.