Are the sun and the moon considered among Allah’s signs (ayat) in His saying, {Say: Was it at Allah and His signs (ayat) and His Messenger that you were mocking} [9:65]? And what is the criterion for His signs (ayat) that it is forbidden to mock?

Scoffing at God's verses means scoffing at the Quran and the religion, and does not refer to the sun and the moon. The hypocrites scoffed at God (at His obligations, boundaries, and rulings), at His verses (His Book), and at His Messenger Muhammad, peace be upon him, when they doubted Muhammad's ability to conquer the fortresses of Sham, and belittled God's ability to help him.