LOL. My youngest does this ALL the time. He wears his underwear backwards more than he wears it as designed. His pants are frequently on backwards, too. His shoes, particularly his summer sandals are almost always on the wrong feet. Most of his shirts are t-shirts (which fit about the same either way, so he doesn't care which way they are) or polo type shirts (which he doesn't like the collar of on his neck, so they are on right).
The underwear--I let him wear it however he wants. I've also bought him boxers (not a great solution, because he's only five and can't tuck the boxer legs into the pants legs and they get bunched up), and boxer briefs (about perfect and usually on the correct way, but expensive and not all have been replaced yet).
If we're at home, I'll mention to him that his clothes are on backwards or inside out, but basically let him wear them how he wants. If we are going out in public, then I do make him turn his shirts and pants around. I either cut out the tags or buy tagless shirts, so that's not an issue for him.
The shoes he puts on wrong, because it is late in the sandal wearing part of the year and they are getting too small on him. He's figured out that if he wears them on the wrong feet that it is easier to get his toes on the floor, which apparently makes it easier for him to toe walk. I actually discourage the toe walking, so I do try to gently push him to keep the shoes on the right feet. I say gently, because if it appears that it is going to be something that stresses him that day, I drop it and let him wear them how he wants. Just being out of the house is enough stress sometimes to cause meltdowns, and I don't want to hasten the start of them when I need to run errands.
I don't recall wearing my clothes not as designed as a child, although I did have to cut out tags and wouldn't wear certain types of material. I also had to be convinced to wear clothing consistently, apparently at a much older age than is typical, because I can remember conversations my mom had with me when I was a small child (but old enough to remember) about keeping my clothes on. I can remember having one pair of corduroy pants that I loved because I would spend long periods of time staring at running my fingers across it. I liked the way it looked, felt, and sounded--stim trifecta-lol
What I did do that was different was pick a certain shirt and wear it everyday. I had one every grade from grades 2-5. I guess before that my mom picked my clothes, and after she never would buy any shirt that I got "that look" about, to keep me from doing that. She'd have to grab it every night when I was taking a shower to run it through the laundry. It also required a trip to the school to explain to them, when I started this, that yes I had other shirts, yes the one I was wearing constantly was washed every day, and no we weren't too poor to buy other clothes. This was the late 80's, so their only real concern was that I wasn't coming to school in dirty clothes.
When the shirts got too holey or too small they all had a mysterious accident wherein the washing machine just...ate them. Tore them to shreds--actually my mom was burning them and presenting me with a scrap of them as evidence of the washing machine's dastardly deed. She drew the line at letting me go to school in a shirt worn so thin that it was see through and full of holes. Which was actually a lie that was probably the least traumatic avenue of getting rid of them. I wouldn't have been able to handle watching them go into the trash and get burned. Even thinking about them being destroyed by the washer led to quite a while of mourning and anger at the washer. By the time I figured out what really happened, I was old enough to understand why it had to be done, so wasn't mad at her.