Donald Keene comments:
[Murasaki Shikibu] dismissed as old-fashioned the kind of unreality present in The Tale of the Bamboo Cutter—the birth of a little girl inside a stalk of bamboo, or Kaguyahime’s ability to vanish at will—and she looked down on other early works that relied on the supernatural for their plots. (source: Seeds in the Heart: Japanese Literature from Earliest Times to the Late Sixteenth Century, Keene, Donald, 1999, pp 477-514):
I am not sure this is drawn from The Tale of Genji. If not Genji, then where did Murasaki Shikibu write about The Tale of the Bamboo Cutter (竹取物語), an equally important classic piece of Japanese literature from an earlier time. I don't know if Murasaki Shikibu ever wrote literary commentary or criticism. Or is this from her early poetry?