Dear Em, Jimi, Janis, etc, rocked. They kicked bootie, so to speak. Karen Carpenter was laid back and mellow. If you happen to like kick-ass music, then what the joke means is that the musican isn't in heaven, he's in the other place. Could be stolen ...er, adapted for folk music:

Imagine an accoustic/trad folkie dies and wakes up in a place to find Woodie Guthrie, Robert Johnson, Stan Rogers, and a host of other departed masters. "Ah," he thinks, "folkie heaven." They bring him a chair and a vintage Martin. He sits in wonder, waiting for the for the incredible music to begin. Peering over the heads of those in front of him, he can see a figure has entered and is standing in the front of the room. Then, as a bubble or two lands on his nose, he hears "OK, from the top - an' a one, an' a two..."