Mederma's main ingredient is Onion Extract. It may help to prevent collagen buildup in NEW scars.
It has not done a good job on old scarring, or keltoids (raised, ropey, hardened skin formed as cuts heal) unless the keltoid has been cut away first.
Since few, if any, circumcision scars consist of keltoids or collagen, I'd leave the Mederma to the snake oil salesman.
Come to think of it, maybe Onion Extract will keep the snakes away.