I'm pretty relaxed when it comes to fashion; I live in (boot-cut, not even skinny) jeans, wear heels only on dressy date nights or for important work meetings, flatiron my hair maybe once a week, etc. But there are some things that I just don't "get."
Clothing that fits poorly: I understand that sometimes you gain weight and don't want to admit it (OH I know) or lose weight and can't afford to replace your whole wardrobe. But when someone is bursting out of a shirt or having to hold up their jeans with one hand, that's a problem. Clearance racks, sales, layaway, raiding your friends' closets...get something that fits!
Designer logos for no reason: I'm not a big designer person, and I really I do not get people who wear labels for label's sake, like a plain, ordinary T-shirt with a designer logo across the front. I totally understand springing for the high style and excellent workmanship of real designer clothes and accessories, but when you're buying that plain hoodie with a designer's or store's name, you've just paid to advertise for them (and probably paid more than you should have, too).
Leggings as pants or worse, opaque tights as pants: Leggings are great, but wear them with something that covers you to at least mid-thigh. Yes, even if you're 20 years old and cute and work out all the time. And if you followed the first rule (which is, if you think "should I wear pants with this top?" the answer is YES) make sure what you think are leggings are actually leggings. If light passes though them when you stretch the fabric or they have feet, they're tights.
Pajamas in public. Why? Just, why? Dispensation for anyone under about 20 and if your clothes are not easily spotted as PJs (there are black knit pajama pants that pass easily for workout wear) but it does not look cute. I love PJs as much as the next person, but they do not look cute in public.
What's your fashion pet peeve?