Some people are good at making up jokes but not tunes. A few people can write moving lyrics for other people's tunes.

Parodies come in all shapes and sizes. Guthrie's Casey Jones song fits our expectation of what a "parody" ought to be, but most parodies today don't seek to mock the original song. Did Guthrie also write the Soup Song to the tune of Bonnie Lies Over the Ocean? It doesn't mock the original song at all. But because everyone used to know that tune, it made it easier to sing the new song. Today, most new parody songs are like jokes: they are best heard once. I usually enjoy them the first time, though often, reading them is as much fun as singing them.

And speaking of the folk tradition, I once got up the nerve to sing "I Need A Sheep", and at the end was asked, "Were you trying to do Scotland the Brave? ...here's how the tune goes..." I steadfastly refused to listen.