Countries do certainly go in and out of fashion, but fashionability has less to do with geopolitics and much more to do with economics and demographics.
Not all
that long ago, Italy and the Vatican City were very fashionable. They're still collected of course, but the oomph has gone out of them. Europa-themed stamps were highly fashionable ... but just try and sell a Europa collection now. Twenty-five or so years ago, people were paying crazy prices for mint
Australian stamps: I know, I was there.
But look at which philatelic threads here and elsewhere attract the most interest: the Classics never go out of style. Someone mentioned Afghanistan as being on the philatelic outer. Well, it is and it isn't. Noone is making money out of modern Afghan stamps - but just offer a genuine first issue Afghan Tiger on cover, and stand aside for the rush of collectors waving money.
India has also been mentioned. Like China, this is a story of economics and demographics. There are more collectors, with more money, in India and China. Never mind that stamp collecting only appeals to a tiny minority. Tiny minorities of the populations of India and China are still big numbers. Then there's the example effect: China and India are simply more prominent in the world, leading others outside to collect these countries as well.
The Classics of these countries always had a following, and so weren't cheap. But the addition of a just a few new collectors has put a rocket under the prices of the Classics. And here is where I come in: when I took up the Indian States back in the 1950s, noone in their right mind collected them. The Classics had a limited following, it's true, but they were generally despised. This stamp of Jammu & Kashmir

(SG 3 - the 1 Anna royal blue watercolour on native paper of 1967)
was catalogued at £450 in 1984. By 1998, it had stretched to £550, essentially on the strength of non-Indian demand from collectors of the Classics. In 2002, it had reached £600, and then it took off. Today, it's a £2,000 stamp - if you can find one.
Prices for the good stuff go up, pretty well regardless. Prices of the common stuff stay common, pretty well regardless.
Some prices of good stuff will go up faster than others, but if you ask me which I'd prefer, $1000 worth of mint recent Australian commemoratives or an equivalent value of, say, early Guatemala covers, the answer's obvious to me.
Of course, I've been using prices as a proxy for interest, but I can't think of a better one.