Here sitting represents ruling. What he sits on can be stated clearly. AT: "rules in the heavens" or "sits on his throne in heaven" (See: [[rc://en/ta/man/translate/figs-explicit]] and [[rc://en/ta/man/translate/figs-metonymy]])
"the Lord mocks those people." Why he mocks them can be stated clearly. AT: "The Lord mocks them for their foolish plans" (See: [[rc://en/ta/man/translate/figs-explicit]])