The moms and dads at DD's school are often just like you are - wet hair, baseball caps, in a rush in whatever they threw on. (We joked that we clean up nicely when we saw each other at an event for one of the teachers earlier this week.)
So if you want to change what you wear, go ahead, but do it FOR YOU, not for them. Frankly, I look at a grown woman with "juicy" across her butt and think how juvenile it is. I don't want her clothes.
What I would do is go to Kohl's or look around online for simple clothes that are easy to put on. Instead of a tshirt, what about one that flows nicely and might hide a problem area? It is just as easy to grab a pair of sweat pants as it is to grab a pair of nice jeans, if you already own them. One of my favorite tops is all dotted and swirly and it's my favorite bc I know when I pull it out of the bottom of the laundry basket, it won't be all wrinkled. I use octopus clips for my hair. I wear a lot of slip on shoes (hey, most days it's enough to find socks for the kid!) either simple flats I bought at Target or my trusty Sketchers Bikers. I have a couple of pairs of "nicer shorts" I got from Target last summer.
A day when I look more "together" is great. And some days I just put my hair in a braid and am just glad to get out of the house before the elementary school bus blocks me in. (FYI, I'm in my 30s, which seems to be in the middle of the ages of the parents in our school.)
And even on bad days, I try to always at least brush my hair, teeth and put on lipgloss that I keep in the car. Not for them but for me. Because I feel better when I'm only a partial slob vs a total one. So if you do nothing else, make sure your shoes are actually on your feet and you're wearing one of your own shirts and have brushed your hair. Even if you take that brush into the car to do it at a red light. Take a few extra minutes for yourself in general.