realistically, if you're not going to care how people perceive you, and refuse to make changes to your appearance because of how you get treated, "because what you're wearing should have no effect on how people treat you", then why not get rid of all social conventions altogether? if clothes are just to cover you and keep you warm, then cutting a hole in an old blanket and wearing it serves the same purpose. combing your hair doesn't serve any useful purpose, you might as well just let it mat up or shave it off completely! why bother ever buying a new pair of shoes, or putting on any kind of accessory? you are already bowing to pointless social conventions if you own more than one change of clothes and if any of them are "decorative" in any way at all, if you shower or have your hair cut, or regularly brush your teeth. you'd still be human, people should still treat you nicely when you walk into stores or meet them on the street.
no one's saying you need to start wearing Gucci or Prada to get the respect of the PTA moms, they're just saying that if you're so self conscious of how you think people are treating you, then there are simple things you can do that are along the same lines of things you already do every day. you don't need to get all gussied up, or spend hours blow-drying your hair, but if you're going to put on regular-people clothing anyway, it might as well be something flattering and put together (and it is more than possible to find these items at the thrift store or on sale)... if you're going to shower and comb your hair, you might as well get a trim and wear it in a neat and attractive style.
no one has ever treated me poorly based on my appearance, and i look quite young and am hardly a fashion-plate. i have gone through multiple style phases, from semi-goth when i was a teenager, to harried, half-awake mama (oh... yesterday?) but i am confident, polite and friendly, and generally following the social conventions of neat, clean dress and personal grooming.