Here "wickedness is sweet" represents a person enjoying doing wicked things. Alternate translation: "Although doing wicked things is pleasurable like tasting sweet food in the mouth" (See: [[rc://en/ta/man/translate/figs-metaphor]])
This represents wanting to continue to enjoy the wickedness as one wants to continue enjoying tasty food by keeping it in the mouth. (See: [[rc://en/ta/man/translate/figs-metaphor]])