Not for the pipes, although they are okay (not to everybody's ears) but for the musical creativity and execution.
Joni was incredible. Not for everyone, that’s true. But I liked a great deal of her work. Her voice as well.
Listen to ‘Ladies of the Canyon’. The left handed guitar chords she chose as an intro. The tempo is one of a person deeply breathing.
Then listen to her three part harmonies. All three vibrato match and are woven into one sound. Not an easy feat. Some artists are fully aware of their natural powers, and are the best judge in how to employ those abilities.