is a duke higher than a countess?

Yes. The order of precedence for the hereditary peerage is:
Duke/Duchess
Marquess/Marchioness
Earl/Count/Countess
Viscount/Viscountess
Baron/ Baroness

Most peerages in the UK are male-only, the woman assuming the titles and stylings of her husband, or taking on the stylings of the peer father. The title of Duke is the highest of the hereditary, non-royal peerage rankings. A duke outranks a countess.

yes

Yes, a Duke is a higher than a Countess. A Duke is the highest Peerage title.

Duke/Duchess
Marquess/Marchioness
Earl/Countess
Viscount/Viscountess
Baron/Baroness

A duke is the husband of a duchess. An earl is the husband of a countess. Because a duke outranks an earl, the answer is yes. A duke outranks a countess.