I thought one reason for a "flying" shark was to spoof some of the mythology surrounding Santa/Father Christmas. It's funny, in America (and I assume in Great Britain) we don't think twice about the notion of airborne caribou pulling a sleigh piloted by a fat beard geezer in a red suit; it's so engrained in our popular culture, we just accept it. But what must it be like for someone from a vastly different culture? Say, any number of societies within Asia. Upon describing such a scenario to a person from that region, he or she would probably just stare at me with concern for my sanity and utter, "That's just silly. The very idea...flying reindeer!"
I figured Moffat meant to hold up a metaphorical mirror and let us view a couple of our cultural icons from a new perspective. I'm not saying that was the only reason for the shark, just one facet.
Sincerely,
Bill