According to Wikipedia article: Joni Mitchell bought herself a ukulele in 1957 (she would have been around 14). She had wanted a guitar, but her mother, with rural roots herself, strongly opposed the idea because of the "hillbilly" image that she connected with the guitar. Before rock 'n' roll, the guitar in rural Canada had been mainly used in country and western music and was still widely associated with that genre. Mitchell eventually obtained a guitar, but she continued to play baritone ukulele well into the early 1960s.
A video for this song: