A few indicators:
1. The site of Nineveh is well known. Excavations there give no indication
that its entire population, including the king, was converted.
2. Nineveh was not "a three days walk across" (3:3).
3. Nineveh was the capital of the Assyrian Empire, and the ruler there was the
King of Assyria. The title "King of Nineveh" (3:6) would be like calling Bill Clinton
"President of Washington".
4. The whole story is pervaded by humorous exaggeration - most notably when all
the _animals_ in Nineveh are to be clothed in sackcloth and cry mightily to God.
I would add that Jonah is not just "a fictional account" but a fictional account
crafted with absolute brilliance, & moreover, an inspired fictional account.