Nineveh was not destroyed for their wickedness.
Jonah was not happy about it. He would not rejoice with God over the many who were saved. (not to mention all the cows)
When the Pharisees asked for a sign, Jesus gave them the sign of Jonah.
The sign of Jonah can mean many things, but surely one of them is that God is gracious and merciful, slow to anger and rich with compassion.
If you read Jonah literally, you know it is about God having mercy on the people (and cattle) of Nineveh. If you read it as a satire or parody, you know it is about God having mercy on Nineveh, the people and the cows.