The main topic, as pohnpei stated, is to show America in two different stages: before the revolutionary war and after it. The way Irving uses Van Winkle's nap to show these differences allows for the reader to see the stark contrast of the before and after, without having to see it gradually over time. 

To me, the main topic is the ways in which American life changed (and did not change) when America became independent.  (And, of course, it's just a fun story.)   Rip goes to sleep before independence and wakes up after it.  We see things like the fact that people are more inclined to talk about politics after he wakes up.  What he sees after he wakes up sort of shows us Irving's view as to how independence affected the US.