The dark side is that attractiveness is a form of power. Having power changes people, especially if one has lived a large portion of their life without this particular expression of power.
Through this lens, excessive enjoyment of one's attractiveness and the impact that has on others can be seen as a form of megalomania; excessive confidence becomes arrogance, self-love becomes narcissism.
In short, it's easy to become an asshole, or mentally unwell.
That seems unlikely.
Most of us could stand to gain some degree of advantage, but I'm pretty there some people whose appearance is unfortunate enough that no amount of grooming, hygiene, physical fitness, or clothing will ever compensate for what fate or genetics has burdened them with.
The best part is, there isn't one single right way. Most people will notice and respect the effort, even if the particular style isn't their first choice.
There might be other things in the way -- emotional barriers, etc -- which may be difficult to overcome. But that is a different matter. And that work would also be very worthwhile to pursue.
I think most people absolutely could benefit, but not necessarily enough to level the playing field with the most gorgeous among us, while for others there really may be nothing they can do to level the playing field against even the most shoddily dressed average looking person. I've got nothing against trying to do the best you can with the hand you've been dealt, and wouldn't discourage anyone from doing that, but when we're talking about the innate bias humans have when hiring I think it's worth admitting that some will always have a considerable disadvantage regardless of their best efforts.
At the risk of sounding arrogant, let me share my personal experience. Over a period of a year in my early thirties, I became more conventionally attractive by: purchasing clothes from StitchFix (a styling service), getting my hair cut regularly by a stylist, adopting a regular grooming routine, and exercising (especially weight lifting) frequently. Prior to this I dressed like a negatively stereotypical engineer with baggy/unstylish clothing, little to no grooming routine, and little to no exercise.
People of all sexes and sexual orientations unquestionably treat me better than they did before. They are more open, more willing to give me their time and attention, quicker to trust me. I’m given favors like extra services (a barista telling me I can cut the line and give her my order directly), or small free things (bartender comping a drink for no reason). And frankly, people just look at me more. I used to feel completely invisible walking down the street, never catching anyone’s eye - not the case anymore.
Because the change happened over a relatively short period of time, and because I spent my entire adult life prior not engaging in these intentional efforts to be “attractive”, the contrast was extremely obvious.
There is a dark side to all this that I’m not going to get into, but I will say that having experienced the contrast I do feel motivated to maintain some level of attractiveness for the preferential treatment.