I'm not sure I'd consider that the best Christmas special ever, but it was perfectly acceptable, and quite enjoyable. As you said, a genre crossover is always a fun novelty.
I don't think Matt Lucas was exceptional, but he did as good a job with that part as any actor would. It didn't seem to be unreasonable to "resuscitate" him from his previous appearance, and the Doctor has previously had companions for a single occasion of several of the previous specials. Ultimately, the companion isn't there to be in the limelight, but as a foil for the Doctor, and a character around which explanations can be woven.