When it comes to appearance, the large majority of people look generic and unremarkable. In order to capture significant benefits from appearance, the main problem is to (a) be noticeable at all, and (b) not in a "wow that guy looks noticeably terrible" sort of way (like e.g. a hobo).

I personally have A Look. Waitstaff at restaurants recognize me the second time I go, and if I have a regular dish they remember it pretty quickly. People at conferences immediately recognize me, even if I don't wear a nametag and we haven't met in person before. Random passers-by on the street regularly compliment my outfit. People dress up as me for Halloween.

But more importantly than any of that, my clothes immediately communicate a vibe about what kind of social role I'm playing.

Almost everyones' clothes communicate that they're role-playing an NPC; they're wearing things which extras with no lines would wear in the background in a movie. The things I wear look like, at minimum, a named character. So people intuitively expect that I will act like a named character - that I will have crazy plans and sometimes even be able to pull them off, that I somehow know things ordinary mortals don't know or can do things ordinary mortals can't do, that I can get away with saying things most people wouldn't say, etc. So they're less likely to push back on that sort of stuff, and more likely to roll (role?) with it.

One thing this all means for you: you probably haven't tested an extreme enough intervention to notice significant results.

It will help get early dates, but it also sets a tone for what they should expect in the future. 

I admire your desire to run experiments here, however I advise against it. I spent too many years not dressing well or worrying enough about my appearance because I thought it didn't matter.

I was wrong. It matters a lot. And not just for romance, but for almost all interactions. Everything you can do to look better is like a general stat buff. There is almost no trade-off to make here; there are only the upfront costs of learning to dress better, exercise, etc. and the relatively minor maintenance costs with looking good (can be as little as 2-3 hours a week).

Short answer: yes.

Long answer: being romantically desirable via being hot is not the be-all and end-all. If someone is mildly attracted to you, you'll have more pleasant interactions with them. I have felt this in both directions for myself.

Then those effects percolate outwards as "status" (in the generic rationalist description of the word). If third parties see you having positive interactions with other people, they'll think more highly of you.

And also, non-autistic individuals (I mean in the actual, clinical sense of autism; the effect I'm describing is actually one of the most interesting and unique features of autism as opposed to just social awkwardness) preferentially ally themselves with people they expect to be high-status. So if you're attractive, since people (on some level which is basically instinctual) will expect you to be have more social cache with others, they'll be nicer to you even if they're not attracted to you, nor have they seen you have a series of positive interactions with people who are.

P.S. I'm unclear from the information you've given whether you actually do have high variance in attractiveness over time. E.g. a "fresh" haircut is only good if the haircut looks good on you. For men it is (famously!) often the case that their freshly-cut hair is unattractively short. I usually find 2-10 weeks post-haircut to be optimal. For clothes, do they fit you well? Do the outfits go together and complement your overall style? The variance here comes from "5-year-old T-shirt and jeans" vs "well-fitting shirt and jacket" not vs "new T-shirt and jeans".

Generally from what you've said I'd guess that you're not getting much natural variance in attractiveness.

Looking good is an initial filter type thing, which shows you put in the minimum required effort. It's a signal that you take care of yourself. Though as @Elizabeth notices, that also sets basic expectations. 

You can get dates and high quality relationships without looking good, but you're playing with a large handicap, which you'll have to overcome with multiple years of high quality friendship or equivalent.

Also, don't trust your own judgment of whether your clothes look good (unless you have exceptional taste, which I doubt from how you're phrasing the question) - ask some <target gender> friends whether whatever you're wearing looks good on you. Even better - ask them to go shopping with you. 

As a girl, it absolutely makes a difference. I'm more likely to get approached if I'm wearing contacts + dress than if I was wearing glasses + jeans and t-shirt. The variance in how people treat you is strongest with strangers (because they have no other information about you to go off of; for example, I would think someone who has a hole in their shirt as more likely to be low-conscientiousness). I stand and sit somewhat non-optimally but I haven't been able to correct this reliably.

Your description of old waterbottles etc. signals that you're frugal, practical, and don't care much about optics, and you probably want a wife with those traits. Seems reasonable for you to keep doing what you're doing if you don't care about her putting a ton of effort into her appearance.

Some more low-hanging fruit for improving your appearance: just don't have any holes in your clothes at all; toss anything with stains you can't get out, and anything with frayed and/or overstretched edges anywhere, and any faded clothes – they always fade more in some places than others. 

You mention posture, and yes, good posture is very attractive, and people will  respect you more, too. Also this is definitely one area where improving does help you "become a better person", or at least feel more confident and "agentic"; at least, that's how it's worked for me.

I think there are two categories of traits, the fungible and the nonfungible.

Fungible traits are money, height, BMI/weightlifting. Them being able to pull you is a proxy for their social status.

Nonfungible traits are your actual contributions, what type of person you are, what role you play in a friend group or room, etc. Includes kindness, humor, emotional intuitions, etc.

When girls realize that my nonfungible traits aren't quite their cup of tea, they say "well at least i scored 5'9" / 6 figures / can pick me up and carry me a dozen feet" and then dump me for someone who's 5'10" / 7 figures / can pick her up and carry her two dozen feet. But if my nonfungible traits are her cup of tea, fungible traits don't seem to do much of anything!

In other words, fungible traits are the fallback when a girl doesn't like you very much. They're literally only worth considering if you assume she doesn't like you as a premise.

I think the answer is that clothing is halfway between, or plays both roles. Clothing contributes a lot to the "my friends will think I pulled well if I bring you around them" factor, so they're like a fungible trait. But clothing is also a vector of self expression, and to many it's a conscientiousness proxy / proxy to how clean your room is which people are screening for in long term primary relationships.

So idk.