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.
The rest to some faint meaning make pretence,
But Shadwell never deviates into sense.
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:
Besides his goodly Fabrick fills the eye,
And seems design'd for thoughtless Majesty:
Thoughtless as Monarch Oakes, that shade the plain,
And, spread in solemn state, supinely reign. (Lines 15-28)

Basic set up:

Mac Flecknoe is a mock epic poem that makes fun of Thomas Shadwell, a rival poet of Dryden's. Dryden—in the lines above—tells us just how dull and stupid Shadwell is. Yeah, Dryden did not pull any punches.

Dryden really does not like this guy Thomas Shadwell. The lines above make it clear that he thinks he's stupid and dull. Dryden's attack on Shadwell shows just how deep some of the literary rivalries ran during the Restoration period—these poets and writers competed with one another and worked hard to outdo each other.

What's more, Dryden's poem shows how critical the Restoration writers often were of their societies and of figures within those societies. The dudes liked to expose people's flaws and to reveal their shallowness and stupidity. Dryden does a pretty good job of it in this poem, that's for sure.

Dryden is absolutely, positively, no doubt about it making fun of Shadwell in Mac Flecknoe :

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.

Whoa, Dryden. Why don't you tell us how you really feel about Shadwell?

Poor Shadwell doesn't come out looking very good. Dryden's satirical wit reduces him to a senseless, dull, stupid lout. Thank goodness Dryden didn't write satirical poetry about us.