How is Zimri an unqualified ruler?
In Absalom and Achitophel, Zimri is described as:
A man so various, that he seemed to be 
Not one, but all mankind's epitome;
Dryden states that Zimri is "stiff in opinions, always in the wrong." Dryden's satirical portrayal of the second Duke of Buckingham as Zimri is well known. In the poem, Dryden hints at Buckingham's womanizing and carousing ways:
But, in the course of one revolving moon,
Was chemist, fiddler, statesman, and buffoon; 
Then all for women, painting, rhyming, drinking,
Dryden paints a portrait of a man who is unstable, untrustworthy and aimless. Buckingham was well known for these traits; he was also dissolute, injudicious and indiscreet.
Blest madman, who could every hour employ,
With something new to wish, or to enjoy!
In squandering wealth was his peculiar art;
Beggared by fools, whom still he found too late;
He had his jest, and they had his estate.
He laughed himself from court; then sought relief
By forming parties, but could ne'er be chief.
The feud between Dryden and Buckingham has been historically documented. Buckingham famously lampooned Dryden's heroic drama The Conquest Of Granada in his satire The Rehearsal, poking fun at Dryden's didactic style and his preoccupation with morally heroic but often abrasive characters. Dryden gets his revenge through portraying Buckingham as Zimri, morally decadent and degenerate. A Dryden hero is ruggedly powerful, uncompromisingly aggressive and morally pristine; Dryden paints Zimri as the complete opposite, and therefore unfit to rule.
Thanks for the question.