Shadwell alone my perfect image bears, Mature in dullness from his tender years. Shadwell alone, of all my sons, is he Who stands confirm'd in full stupidity. (15-18)
Dryden makes it clear how he feels about Shadwell from the beginning of the poem. We can tell that Dryden's sole purpose here is to belittle his rival, using his own wit to poke fun at Shadwell's lack thereof.
Thoughtless as monarch oaks, that shade the plain, And, spread in solemn state, supinely reign. Heywood and Shirley were but types of thee, Thou last great prophet of tautology: (27-30)
"Mac Flecknoe" is littered with references to the seventeenth-century English literature scene, as Dryden name drops writer after writer over the course of the poem. Some of these writers he views favorably. Other writers? Eh, not so much. But even the worst of the worst, Dryden argues, like Heywood and Shirley, are still better than Shadwell.
Like mine thy gentle numbers feebly creep, Thy Tragic Muse gives smiles, thy Comic sleep. With whate'er gall thou sett'st thy self to write, Thy inoffensive satires never bite. (197-200)
Dryden bashes Shadwell's writing ability throughout "Mac Flecknoe," discrediting his work as ineffective, cheap, and supremely bland. Here Dryden pokes fun at Shadwell's satires, which is particularly effective considering that "Mac Flecknoe" itself is an extremely successful, totally offensive and biting satire.