It is certainly a solemn day, but there seems to be a general view that it is also sad. This is probably because Yizkor, the memorial service, is a leading part of the day, though Yom Kippur has nothing like the sadness of Tishah B’Av.
Even the ministering angels are puzzled by Yom Kippur (Rosh HaShanah 32b).
Aware that it is a day with a festive character, the celebration being God’s forgiveness of our sins, they wanted to know why we do not recite the Hallel as on other occasions of celebration (Shulchan Aruch, Orach Chayyim 584:1).
The Talmud records this dialogue: “Master of the Universe,” said the angels, “Why do Israel not chant psalms of praise before You on Rosh HaShanah and Yom Kippur?”
God replied, “Can it be that the King should sit on the throne of justice with the books of life and death open before Him, and Israel should chant hymns of praise?”