A fine and detailed study on Mitchell’s great song from her masterpiece album, “Blue”: www.washingtonpost.com/arts-entertainment/2018/12/07/how-thoroughly-depressing-joni-mitchell-song-became-blue-christmas-classic/
My impression reading it was that there were no sad or melancholy Christmas songs before it, but “I’ll Be Home for Christmas” and “Blue Christmas” are two that immediately come to mind. And later on songs like “Last Christmas” have shown that the holidays can be sometimes difficult.
Read the piece though. Lots of good commentary by great singers who have covered it, as well as what it really means.