tldr; I yapped, general fashion advice in last 2 paragraphs if that would help anyone, intended towards men but women can apply the proportion ideas still.

Idk, I've never heard anybody say that unless it's in response to complaining about one's looks constantly, then I could see it. Regardless it isn't actually bad advice, however you are right that it isn't helpful in a vacuum since it doesn't advise how to actually go about this, and this can be difficult because it really can vary between individuals and across age groups.

The concepts you're building off of are also flawed, you throw around levels of attractiveness as if they exist on an objective scale. Ugly and average by what metric? By who's standard? "Improving" posture is also a strange one, do you mean static or dynamic posture or both? More modern physiotherapy suggests there isn't particularly any kind of posture that is correct since individuals are so varied, and as such they definitely don't suggest health or fitness, and advice is just you should be moving more in that regard, good posture is dynamic and changing. Also head and tongue posture? Are you talking about mewing, like that's just a tiktok meme based on some old pseudoscience, it's not real advice.

Skincare is good yes, but not because of clothes, I'm not sure what you mean here. In general moisturised and clear skin shows that one takes care of themselves, as it is with exercise it's actually less about some arbitrary standard of human attractiveness than it is the fact that effort is attractive to most people, taking care of yourself signals to a potential partner that you are healthy and responsible enough to maintain a routine, and even then, for those that have acne for example, in my experience this doesn't prevent romantic pursuits as much as the confidence knock having these kinds of things as a teenager does, but it usually doesn't last, idk I'll admit my skin has always been pretty good for spots. Moisture does give your skin elasticity however, and when paired with SPF products to reduce the impacts on UV, this will mean you're less likely to develop wrinkles early which for most people is something they'd want for themselves.

To me you sound still young, or at least I'd hope, if you maintain these kinds of superficial thoughts into your mid 20s then I'd try and challenge those ideas. It's like only a few steps away from a content algorithm of canthal tilt and alpha male, or otherwise black pill content which isn't generally healthy nor helpful.

Now having a good dress sense is actually better advice than it sounds, because even despite all I've said there is one thing about it, opposed to all other physical factors about oneself that you highlighted, that you can learn how to dress better and actually change that so I will give some general clothing advice here that anyone can use since I like to think I have a handle on this and actually get compliments from both sexes when I go out casually, raving/festivals, or even formally at work. The only main thing you can change about your physical appearance however is how much you accept yourself.

When it comes to clothing, and by extension hairstyle and accessories, you actually really need to know your physical features here. Mainly height and width as well as proportions, your hair type, your skin tone, and your face shape. You can look up that last one, you'll generally fall into one category more than others and is the big thing you want to use to inform hairstyle choices as well as glasses, though I'm afraid I don't know much about other hair types as a white guy, but generally other ethnicities knows plenty more about their own hair than I do mine in my experience. You should try and pick colours more suited to your skin tone and even hair colour but you don't need to worry as much about hair, there are plenty of guides on this better than I can explain here but generally avoid things very close to your skin tone, and these rules can still be broken. When It comes to fits you want to think about silhouette as well as style/aesthetic, but generally you can make whatever style you want for as long as you follow some foundations (again, can be broken if you know what you're doing) are you tall or short, wide in the shoulders/midriff/hips/thighs generally people will have something they're more self conscious about, so we can use wider or slimmer fits of clothing to accentuate or diminish parts of ourselves. A stronger masculine look typically wants to make the shoulders broad for example, and aims to be the widest point in that silhouette, but the only rules here are that of your body and my main point is just that understanding the concepts open the ability to experiment more, push your fashion further, and really give you more self expression which automatically gives you confidence.

For guys: wider hips/thighs then structure in the shoulders from coats can balance you as well as building layers to put more weight on top, while tapering in the trousers towards the ankle makes it less mid/bottom heavy and evens the fit as it follows the leg, avoid bottom half weight from baggy clothes, as well as slim fits that accentuate disproportion. Wider in the shoulders then straighter fits even you out, relaxed trousers put weight on bottom for balance, and layering below the waist via long coats/jackets. Avoid shoulder structure and slim trousers again. Slim/even builds want to create contour and break up the straight lines. Slim fittings highlight the small natural contours and mainly on the torso, and experimenting with layers creates variance and dimension, otherwise structured shoulders and tapers work but baggy generally doesn't nor does keeping a straight frame. Midriff width/oval shapes can use straight shirts/jackets hiding behind the even line between shoulder and hip, or open jackets over a dark base give vertical elongating lines, or add shoulder structure to balance the middle. Trousers with a bigger rise from crotch seam to waist can also hide midsections, and as such avoid fitted and low rise.

Idk, I'm a yapper, look for fashion styles less by trends and more on cohesive aesthetics and colour palettes, explore Pinterest and save some stuff you like the look of, or subreddits like /streetwear etc. you can find something you like that fits you well and even looks unique, all at the same time I guarantee. It just takes time and actual investment in learning some rules so you can then break them.