Would December be a good time to visit Taipei?

Well, the average temperature of December is about 16 C. It might rains a lot then but usually not heavy. Anyway, it's still siutable for your traveling then because it's cool. Actually, the best time for Taipei trip is during November to April in the next year.
By the way, the typhoon season of Taipei is during June to October, so don't worry about the typhoon in December. As for the earthquake, it does not happen frequently and that would not be a problem.