These two lines are taken from Mac Flecknoe (written ca. 1678, published in 1682). Mac Flecknoe, a verse mock-heroic satire by John Dryden (1631-1700), is a direct attack on Thomas Shadwell (1642-1692), another prominent poet of the time, who is appointed the heir to a kingdom of poetic dullness.
Mac Flecknoe is the outcome of a series of disagreements between Thomas Shadwell and Dryden. Their disagreements were political, religious and literary.
2) Religion: Shadwell offended Dryden when he satirized Catholic and Anglican priests in a play (The Lancashire-Witches, and Teque o Divelly the Irish-Priest, 1682). Dryden was considering the idea of converting to Catholicism, which he did in 1686.
3) Literature: Shadwell and Dryden disputed about who was the best writer: Shakespeare (Dryden) or Ben Jonson (Shadwell).
As a consequence, this Juvenalian satire, which is an ad hominem attack, is also a piece of literary criticism. The lines “But Shadwell’s genuine night admits no ray, / His rising fogs prevail upon the day” (ll. 23-24) refer metaphorically to Shadwell’s lack of brilliance, to his stupidity (according to Dryden). For Dryden, Shadwell, whose poetry was largely dismissible, was the Lord of misrule. In the poem his only attributes were indeed a mighty mug of potent ale and a crown of poppies.