Satire-Mock-Epic (in Heroic Couplets)
The poem is written in rhymed couplets of iambic pentameter, also known as heroic couplets. Whew, that's a whole lot of terminology to take in at one. To see what we mean, take a look at lines 11-14:
And pond'ring which of all his sons was fit
To reign, and wage immortal war with wit;
Cry'd, 'tis resolv'd; for nature pleads that he
Should only rule, who most resembles me:
Notice how each line is comprised of five iambs—that's just the fancy literary term for an unstressed syllable followed by a stressed syllable (it makes a sound like daDUM). That's the iambic pentameter part (penta- just means five). Of course the entire poem is made up of rhymed couplets, which combined with iambic pentameter, gives you your classic heroic couplet.
This form is typical of epic poetry, but "Mac Flecknoe" is no epic. It's a satire, employing the meter, rhyme, and elevated language of the epic to make an ironic point. You could call it a mock-epic, or mock-heroic, a unique poetic style that Dryden popularized in English Restoration literature. Whatever you call it, you better also call it groundbreaking and influential. Several decades later, legendary English satirist Alexander Pope would develop this form even further.