While still more people gathered in the crowd, he said to them, "This generation is an evil generation; it seeks a sign, but no sign will be given it, except the sign of Jonah. Just as Jonah became a sign to the Ninevites, so will the Son of Man be to this generation. At the judgment the queen of the south will rise with the men of this generation and she will condemn them, because she came from the ends of the earth to hear the wisdom of Solomon, and there is something greater than Solomon here. At the judgment the men of Nineveh will arise with this generation and condemn it, because at the preaching of Jonah they repented, and there is something greater than Jonah here. (Lk 11:29-32)
When Jesus said that this generation is an evil generation he did not mean to imply everyone in this generation. He was intending only those who were seeking for a sign. Those who sought for a sign, (and not believing in his word) were the members of evil generation. He also said that no sign would be given for those who sought for a sign.
There were three types of signs: one was a proof sign which convinces the observer about something and the other was a prophetic sign which resembled the situation announced the third was a reminder sign. What Jesus meant was the second type: prophetic sign. This sign only resembled the situation which Jesus was speaking about.
Jonah was a sign to the Ninevites. Jonah’s life and his preaching were signs to the people. What Jesus meant was that the people of Nineveh accepted the teachings of the prophet and converted themselves and returned to the Lord. Now Jesus was like Jonah and he went about teaching to the people. But the sad story is that the people have not repented. Jesus is greater than Jonah.