Some beams of wit on other souls may fall, Strike through and make a lucid interval; But Shadwell's genuine night admits no ray, His rising fogs prevail upon the day: (21-24)
In order to elevate the poem from a cheap critique to a hilarious and imaginative satire, Dryden drops some seriously clever insults. Here he uses metaphor to go after Shadwell's intellect. Throughout "Mac Flecknoe," Dryden employs his exceptional command of poetic conventions to very creatively, very literarily, basically just call Shadwell a stupid idiot.
So Shadwell swore, nor should his vow be vain, That he till death true dullness would maintain; And in his father's right, and realm's defence, Ne'er to have peace with wit, nor truce with sense. (114-117)
Dryden takes great care in the poem to note Shadwell's lack of wit and cleverness, which is only made clearer in the context of Dryden's own supremely witty and clever writing. Described amusingly as terminally dull, and perpetually at war with wit and sense, it's clear that Shadwell does not have the intellectual upper hand.
And when false flowers of rhetoric thou would'st cull, Trust Nature, do not labour to be dull; But write thy best, and top; and in each line, Sir Formal's oratory will be thine. (165-168)
Dryden depicts Shadwell as a heroic figure in "Mac Flecknoe," an excellent satirical move. All great heroic figures have a primary virtue, but for Shadwell, ironically, his "virtue" is dullness.
Where sold he bargains, whip-stitch, kiss my arse, Promis'd a play and dwindled to a farce? (181-182)
Shadwell's writing is completely un-literary, Dryden argues, devoid of substance and taste. Dryden mentions several of Shadwell's not so clever catchphrases, which he thinks are only used to grab a cheap laugh.
Nor let thy mountain belly make pretence Of likeness; thine's a tympany of sense. A tun of man in thy large bulk is writ, But sure thou 'rt but a kilderkin of wit. (193-196)
At least Dryden found a clever way to tell Shadwell he's fat and stupid. This cunning insult is one of many in "Mac Flecknoe"—where the subject matter is hardly brilliant, but the way it is presented is pretty darn creative and memorable.