What is funny about the first lines of the story of Bacchus and Pentheus? How does Tiresias fit in?
As you state Tiresias is the seer that introduces the story of Bacchus and Pentheus. Their interaction is one filled with tension and from one ironic perspective, there can be said to be humor. Let me explain.
Up to this point, Tiresias' fame has been spreading. While he is blind, he had extraordinary foresight. All knew this and respected him, except Pentheus. Tiresias told Pentheus to worship the new god, Bacchus. However, Penthesus refused. Tiresias stated that if the did not worship Bacchus, Pentheus was be killed, torn apart and spread out over the woods. As one might expect, Pentheus did not believe him and opposed Bacchus. In time, as Tiresias predicted, Pentheus was killed.
There are two humorous points there. First, Penthesus makes fun of Tiresias' blindness, but it is really Pentheus who is blind. Tiresias sees perfectly. Second, Pentheus would be killed by his own aunt and mother as they mistake him for an animal. This too is somewhat humorous, because in Ovid's Metamorphoses, many people are turned into animals. However, in this case, Penthesus is not turned into an animal and yet he is killed as an animal.