Look...
Many people will probably answer something pre-defined such as "beauty is in the eye of the beholder", that beauty is subjective and that its standards change throughout time.
In part they are true, in part they exaggerate or misuse the statement of subjectivity.
They are true in the sense that it is visible throughout history that there have been, indeed, different ways of defining aesthetics. Until 18th or even 19th century it was a sign of social prowess to be chubby and "fat" since it showed that you could eat a lot and were actually well off. See a few old portraits and you'll notice that even Helena, the most beautiful woman of ancient times, is depicted in a way that most people in the 20th or 21st century wouldn't consider beautiful.
Often you get different standards for beauty also within the same period of time (mainly across geographies), but there is always one ideal, one model of human beauty towards which all other different forms of beauties somehow converge.
What can be said for
sure is that social standards for beauty are definitely more generalized than individual tastes: for instance, I might like a girl with a slight asymmetry in her face and be really attracted to her, while the rest of society actuall considers her ugly (better for me, lol, no competing suitors!).
However, a specific set of
social standards in beauty -
as in anything else - prevail in determining the effect on the psychology of the people having it (depending on how exposed they are).
And here I think that being beautiful ("beauty" in the sense of beauty standards by the given society at a given century) does help.
But it usually isn't anything that comes from within. It's a kind of
self-awareness about how the rest of society perceives one. When you
know that others think you are beautiful (and better, if you hear it), your "ego" is solidified.
And whatever strengthens your "ego" will possibly assist you in conducting a better life.
This doesn't mean that
all beautiful people (again, with beauty meant in the social sense) get ahead in life only because they're aware that they are beautiful, nor does it mean that there aren't other valid sources of self-awareness and ego-strengthening.
And most importantly, this doesn't mean that all beautiful people actually ever become self-aware of how beautiful they are: there were many girls I knew who I personally considered very pretty, but from the way I spoke with them it seemed like they believed to be ugly as hell...
So, not all beautiful people get ahead in life. The key requirement is
self-awareness.
With regards to composition... ehm... many really great succesful composers were/are actually bad-looking in my opinion. I don't think that anybody cares about how they look; people rather care about how they
sound.
There was a period in which I was very fat and people just laughed at me. Still, I was good at a few things for which people actually praised me at that time
But perhaps you are also shooting music videos' or something like that...?
I didn't really understand the direct connection between physical appearance and success in music...
In any case, it's all just random social dynamics. Trust me, in a few decades there'll be totally new standards and stereotypes for beauty, so I don't think it isn't worth basing your musical success on your physical appearance.