Let everyone turn from his evil way and from the violence that is in his hands. Who knows? God may turn and relent and turn from his fierce anger, so that we may not perish.”Jonah 3:8-9
Repentance starts with recognising wrongdoing, can’t repent from sin if we don’t see ourselves as a sinner.
Jonah comes with a message against Ninevah (ch1). This is probably not all Jonah said but God’s opinion of their evil is certainly part of his message. The Ninevites hear they’ve got 40 days before God overturns the city and they believe God. This isn't softly softly approach, strong language is used - evil/wickedness.