That makes no sense. Speaking as a prophet for God, Jonah told the Ninevites in 40 days Ninevah will be overturned as that was God's plan for Ninevah because of its wickedness.
Since the Ninevites believed God, they declared a fast and all put on sackcloth. When the King of Ninevah heard the news he too took off his royal robes, covered himself in sackcloth, decreed a fast, prayer and to turn from their evil ways.
God saw their ways had changed, had compassion on them and didn't bring on the destruction he threatened through Jonah. God changed his mind because the Ninevites turned from their evil. That doesn't make Jonah wrong, not at all, and I'm really surprized to see you make that argument.
Context is everything.