I agree with Bill about the sarcasm. Jim was not so much interested in the party because he thought it would be good, but specifically because he thought it would be a crazy mess like most of his other experiences with Dwight’s life (like the B&B he stayed in).
I also agree that because Jim is expecting to leave the office, he may have been a little saddened that he would more than likely not be keeping in touch with Dwight and thus miss out on future craziness, so he wanted to have a last bash.
However, I think that there were a couple of more points that explain his behavior.
He was hyped up about his trip to Philawhatever and the prospect of a new (and presumably more entertaining, not to mention lucrative) job. The endorphins and adrenaline would have made him a little less rational and a lot more excitable. Plus, he probably had a couple of egg-nogs.
He knew that leaving the office and Pam to jet off to Philly at Christmas time was pretty bad timing and he may have been perceived as selfish or more interested in work than friends and family, so by getting everybody wrapped up in Dwight’s craziness, he could distract them from his absence.
That said, I think Jim really did get caught up in the festivities, especially when he found a way to arrange things so that he could go to the meeting, but also stay for the party. This is probably due to his realization that his life is going to change and he will not be seeing these people whom he has grown fond of over the years. This nostalgia and sadness caused/allowed him to drop the sarcastic pretense and genuinely enjoy his last Christmas party at the The Office.