I suspect there was a lot of crossover versus the regular folk singer-songwriter pool in the olden days. Nowadays, if you're talking "mainstream indie", the crossover is probably nil -- joke songs about Star Trek don't exactly code indie cool even in today's superhero-charged media context; and Lonely Island types who do similar things in the mainstream start off there and don't associate back to geekdom necessarily. However, when the fandom is popular, the filk scene can be big. Wizard rock (Youtube it) is/was a proper insular underground with bands that toured and everything.