If someone is being scornful and mocking in a humorous way, call her sardonic. If you want to write comic sketches for late-night talk shows, work on being sardonic.
Sardonic comes from the Greek adjective Sardonios, which actually describes a plant from a place called Sardinia that supposedly made your face contort into a horrible grin...right before you died from its poison. The Greeks used sardonic for laughter, but we only use it when someone's humor is also mocking or ironic.