I think Jesus is telling us to mourn for the unrepentant wicked on the day of His Second Coming. Scriptures and apparitions indicate that Christ's Church is going to go through a death and resurrection on her way into Messianic Reign. In the Third Secret of Fatima, Catholic Clergy mourn for all the dead on the way up Mt. Zion. This reference in the Third Secret, is relating to Jesus telling us to mourn for the unrepentant, not Him, on His way of the Cross. In the Third Secret, we see the Church suffering on Her way into Christ's Kingdom Come, The New Jerusalem, Mt Zion, on earth, as Jesus suffered on His way of the cross to become King and Ruler over the New Jerusalem, Mt. Zion, on earth. When Angels of the Apocalypse, hurl the unrepentant wicked into hell, and the Wrath of the Lamb blows through, putting all the unrepentant to physical death, at Armageddon, these are the ones Jesus tells us to mourn for.
Christ's Kingdom Come, in the New Jerusalem on earth, will be like Easter Sunday. The Church is going to go through a death and resurrection, as she transforms into Christ's Kingdom Come of the New Jerusalem, Mt. Zion, on earth. With Jesus Ruling the world, it will be grand, like Easter Sunday. We will go into the ‘golden era of mankind’, an era of peace on earth, that the Blessed Mother has promised us.
“J.M.J. The third part of the secret revealed at the Cova da Iria-Fatima, on 13 July 1917.
…Other Bishops, Priests, men and women Religious going up a steep mountain, at the top of which there was a big Cross of rough-hewn trunks as of a cork-tree with the bark; before reaching there the Holy Father passed through a big city half in ruins and half trembling with halting step, afflicted with pain and sorrow, he prayed for the souls of the corpses he met on his way; having reached the top of the mountain, on his knees at the foot of the big Cross …
JESUS, THE DIVINE MERCY, by St.Faustina
The devotion to Jesus as the Divine Mercy is based on the writings of Saint Faustina, a Polish nun who wrote a diary of about 600 pages recording the revelations she received about God's mercy.
Jesus looked at me and said, Souls perish in spite of My bitter Passion. I am giving them the last hope of salvation; that is, the Feast of My Mercy. If they will not adore My mercy, they will perish for all eternity. Secretary of My mercy, write, tell souls about this great mercy of Mine, because the awful day, the day of My justice, is near. (Diary - Divine Mercy in My Soul, 965)
The Blessed Virgin Mary from the Diary of St. Faustina (635)
... you have to speak to the world about His great mercy and prepare the world for the Second Coming of Him who will come, not as a merciful Savior, but as a just Judge. Oh, how terrible is that day! Determined is the day of justice, the day of divine wrath. The angels tremble before it. Speak to souls about this great mercy while it is still the time for [granting] mercy. If you keep silent now, you will be answering for a great number of souls on that terrible day. ... (635)