While I am in the minority, I will still voice my objection.

It is Christmas, not Halloween, not just some office get-to-gether...but, Christmas. Accordingly, I would go with something a little more festive and not likely to offend anyone (even at a vet clinic, you might find a person or two that thinks Christmas is Christmas, the birth of Christ).

Even though I love cats, and I love jokes, I wouldn't have wanted someone to make me a litterbox cake for my bridal shower...it just doesn't fit the occasion, IMO.