I would say that Wimbledon have quite a middle class fanbase, who are fairly affluent for the size of the club.
The size of the club is obviously very small in comparison to others in England though, and especially in League One they are one of the underdogs due to their rapid rise through the leagues (5 promotions in the first 9 seasons).
But because it is supporter owned and run by the fans there are lots of people who volunteer their time for free, and who donate all they can financially towards the club. This doesn't necessarily make them affluent though.
Wimbledon as a town is very average. However, there are areas of Wimbledon which are much richer, for example where the tennis tournament is hosted. Lots of places in London have this same balance of richer and poorer areas.
Where Wimbledon are currently building a new stadium - on Plough Lane, right near the site of Wimbledon FC's old ground that is now demolished - it is a more working class part of the town with lots of industrial areas.
So I would say that most of the fans are middle class or working class and it is a very friendly and welcoming football club.
In the current stadium we have 3 bars and nowhere that makes lattes if that helps you make your decision!