How is Dryden's Mac Flecknoe a mock-heroic poem?
Dryden was the greatest master of his time in the art of the mock-heroic poem. These follow the form of classical heroic poems written by early masters, specifically by Homer as Illiad and Odyssey are the standards from which mock-heroics are constructed: The elements remain the same though they are employed to mock with satire and parody rather than to praise.
The form, in imitation of the heroic standard, begins with a tribute to the Muse inspiring the poet to write. In Dryden’s mocks, the muse may be the human who inspired him to wax satirical, like Carlyle for The Rape of the Lock. However, for Mac Flecknoe, Dryden's inspiration seems to have been the contemplation of "Fate":
All humane things are subject to decay,
And, when Fate summons, Monarchs must obey:
In mock-heroics, other formulaic standards are followed, such as the ritual dressing for battle, processions of valor, calling upon the aid of gods, and victory celebrations. In Flecknoe, the ritual dressing, such as Achilles (Homer) and Belinda (Dryden) were privileged to, seems to have been substituted by a ritual undressing as Flecknoe's successor to "Emperor of ... Prose and Verse" has his demerits categorized and satirized:
Sh—— alone, of all my Sons, is he
Who stands confirm'd in full stupidity.
In imitation of Homer, Dryden includes references to Greek gods and mythology as in his reference to Arion, the mythological Greek poet who was borne across seas by dolphins. Dryden concludes his mock-heroic with a celebratory calling of the Homeric-like champion to arms where Flecknoe passes the mantle and departs--actually, in a Biblical allusion to Elijah, he departs and then the mantle falls upon Sh---.
And down they sent the yet declaiming Bard.
Sinking he left his Drugget robe behind,
Born upwards by a subterranean wind. 
The Mantle fell to the young Prophet's part,
With double portion of his Father's Art.