Sackcloth is a covering. The speaker says, “I clothe the heavens with blackness, and I make sackcloth their covering” (2 Nephi 7:3).
“Instead of a stomacher, a girding of sackcloth; burning instead of beauty” follows “instead of well set hair, baldness” (2 Nephi 13:24). The Lord says, “except they repent in sackcloth and ashes, and cry mightily to the Lord their God, I will not hear their prayers” (Mosiah 11:25).
Nephi saw that the people “had repented and did humble themselves in sackcloth,” and he cried again unto the Lord (Helaman 11:9).