When God described Nineveh as being the great city, He was not considering its ignorance and sin, but looked with great joy at its profound repentance as Nineveh was quick in responding to God's word.
When Lot warned the people of Sodom of the Lord's displeasure, they scorned him, and "to his sons-in-law he seemed to be joking" (Genesis 19:14) whereas the Ninevites listened with utter seriousness to Jonah and responded quickly to his word. The word of the Lord was fast, bearing life, efficacious and sharper than a double-edged sword.
The fruit of their repentance was their humility, fasting, wearing of sackcloth and crying out to God. Their repentance was sincere in every meaning of the word, serious and from the heart, resulting in everyone turning "from his evil way and from the violence that was in his hands." (Jonah 3:8)