A lot of their hits are technically covers. "I Love Rock n Roll" was originally written by a UK band called "The Arrows".
I've heard George Thorogood's playing described as "Burgers and Fries". That's a pretty apt description of Joan's playing. Not pretty or fancy - just straightforward and satisfying. The "Melody Maker" style of guitar she plays was introduced by Gibson to be the absolute cheapest thing they could make. Of course, now you can buy a Joan Jett Signature model for a little over $1000.