Upvote:3
The festival site is not accessible by train, so you have to take a bus (as indicated on the "Access" page of the festival's website), which will not be covered by your JR Pass.
By the way, note that Kawaguchiko station is not on a JR line, so trains to Kawaguchiko are not fully covered by your JR Pass either (specifically, the fare for the section between Otsuki and Kawaguchiko must be paid).