This is a great question. It is best to start with a definition of epic and then move to the genre of a mock epic.
An epic is a long work in poetry that tells the story of a hero and his struggles; there is usually a journey and great battles. It is also usually written in dactylic hexameter. The most famous epics are Homer's Iliad and Odyssey, and Virgil's Aeneid. In the Iliad we read of the great hero, Achilles and the battle between the Greeks and Trojans. In the Odyssey, we read of Odysseus and his long journey back home and his battle with the suitors. In the Aeneid, we read of Aeneas and his call to find a new country for his people.
A mock epic is something that has the feel of an epic, but the content is pointedly not heroic. The perfect example is the poet Ovid. The Metamorphoses is written in dactylic hexameter, but the content is not about a hero. Often times it is about the amorous adventures of the gods as they seek to seduce human women.