Missy, understood. I don't own any of Joan's records myself because, like I said, I'm never exactly been a huge fan of her myself. To my ears she was always a bit too folkie (in the traditional sense) but I have heard some stuff of hers over the years that I've liked. As for her ratio of original material vs. covers, I haven't done the math but I'm thinking Emmylou Harris' work might weigh out somewhere close to that as she's probably known more as an interpretor of others' material than a songwriter.
Back to Joan, here's her cover of Ryan Adams' "In My Time of Need" from Dark Chords From A Big Guitar:
....and just for comparison's sake here's Ryan performing a live version of the song: