Just adding to what's already been said: basically, what's considered beautiful depends by class stratification as well as what resources are available to each group. The classic example is older ideas of fat = good, as having extra body fat was an indication that your family was rich enough to afford surplus food, meats and other expensive items, etc. In the modern western world, being thin is a sign of control and a way of suggesting that you have so much money, you can afford to keep your body just on the starvation line, as if there were a crisis, you would be able to purchase food regardless. &c.

There's also the problem of importing beauty standards from another culture and internalized colour hierarchies. Colonisation privileged those with lighter skin and led to plenty of "mixed" children, so that several generations later, there are plenty of pale-skinned, blue-eyed people born below the Northern Hemisphere. Shocking. If you're born into a lower class (economically) but fit the beauty ideals of a higher class, you're going to have an easier time moving up the hierarchy, as you'll inherit all the privileges that come from fitting the ideal.

"Beauty" is always a social construction and determined by the money and resources of the highest ranking. For women, that means nothing is ever good enough and the FDA has given up regulating cosmetics, because if they had to prosecute the false claims by advertisers, they'd be backlogged for years. Naomi Wolf talks about this in-depth in The Beauty Myth, as well as how this is a great way to suck any disposable income away from women (so even if you rise a class economically, you gain new responsibilities -- such as keeping up with the latest fashions, haircuts, makeup, etc., all of which take a huge chunk out of your paycheck).