Although better nutrition undoubtedly makes people more attractive -- for example by giving them enough vitamin A and the proteins and fatty acids needed to maintain healthy skin and hair -- this is only a minor improvement on the pre-existing genetic endowment. And better medicine certainly plays little or no role at all in enhancing people's looks simply because most medicine was harmful until the 20th C.
What could have so altered the genetic blueprint for outward appearance that a population looks a lot better as it grows richer? Note that I'm not comparing richer vs. poorer societies at a moment in time -- those differences could reflect all sorts of other differences -- but how a given group like the Scottish become more attractive over time as their material welfare rises.
There could be many reasons for greater selection for beauty in more prosperous environments, but a very simple one is the disappearance of the looks-vs-money trade-off that men used to face when choosing a wife. Dowries -- material stuff given to husbands by brides -- were routine until fairly recently, surviving into the 19th C. In the poor, Malthusian world that humans lived in before capitalism lifted all boats to comfortable levels, a key factor in a husband's genetic success was having sufficient material wealth to make sure his kids could survive and thrive. So how much dough the wife would bring to the household was a big deal in his choice of mate.
However, once the average man becomes incredibly wealthier and everyone better fed, the dowry that the wife would bring would not contribute that much more to taking care of their kids' necessities during childhood. Therefore he will choose less on the dimension of the woman's wealth and more based on other still important factors such as her looks (a signal of good genes). The magnitude of this selection pressure is going to be pretty big because it's the one that men naturally obsess over the most, and only something like a compelling looks-vs-money trade-off will dam it up. That means that even over 5 to 10 generations, people will become noticeably better looking as cuter women are chosen as wives / mothers.
Overly romantic social scientists often characterize this transition as one that finally frees up the human heart to choose based on true love rather than mere material wealth. It may do that -- but more importantly, it makes it possible for guys to almost entirely vote with their cock. This unbridled lust has done more to make the average person more attractive than have all the feeble endeavors of do-gooders to secure a toothbrush and balanced diet for every child.
By preferring the support of a comely to that of a wealthy wife, he intends only his own Darwinian fitness; and by directing that marriage in such a manner as its produce may be of the greatest pulchritude, he intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention. Nor is it always the worse for the society that it was not part of it. By pursuing the good looks of his own children he frequently promotes those of the society more effectually than when he really intends to promote them.