When I visited the monkey park on a cold day in winter there were only a couple of tourists and I did not see any food throwing either. Also tourist facilities were rather minimal (a tiny hut with a heater) so I don't quite share rpbourne's impression. I actually quite liked the monkeys.
However in early December you might not have snow yet (and from what I read the experience is much worse without snow). Also, you have very little time in Japan for such a large detour for maybe 2 hours of monkey watching. If you go it would make sense to allow maybe 2 days or so and combine it with Matsumoto, the Kiso Valley, skiing or something else in order to justify the detour.