It's not the monarchy that's popular, it's the celebrity status of Will and Kate, who on any given week, have their faces plastered over every entertainment magazine, gossip rag or supermarket tabloid in the world. So sure, there's appeal for them, but roughly on par with that in the US or Germany or any other place, republics included.
Opinion polls repeatedly back this up (Google 'monarchy opinion polls canada' if you like), but for proof, compare their appeal to that of the umm ... less attractive and glamorous ... Charles and Camilla.
It never fails to amaze me that people use the popularity of William, who isn't even next in line, as proof that the monarchy is popular, yet the unpopularity of Charles, who is next in line, is no indication of its unpopularity.