Jesus names the source of baptism then asks where it comes from. As with so much of Jesus's humor, we have to imagine the pause that sets up the punch line. The pause after "where is it from?" invites the answer
"John."
In using the term, "heaven," Jesus didn't make this about divine power alone. The term also means "universe." So the question can be taken as a question about the source of one's power in general: Does it come from the larger universe or from ourselves. It can also be taken as a contrast between power coming from the natural world of God or from the artificial world of society.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.