Mike,
I'm not sure they are open daily at Christmas - you might want to check the calendar. SFMM is open only weekends for a good part of the year. That said, I'd think the park would be a ghost town between Christmas and New Years. SFMM doesn't get crowd patterns the same way as Disney. If it was a busy day, Six Flags has the Q-bot system to ensure you get on everything - as long as you have the money.
So, I just checked the calendar, and the last 2 weeks of December they are pretty much open daily. I don't know if anyone around here has been there during this time and can comment on crowds. Personally, I just don't see people goiong to SFMM during their Christmas break, but they wouldn't be open otherwise... Even so, the hours are 10-6 which doesn't sound like they expect big crowds.
William is probably best suited to answer this question.
Thanks for the response... One would think that the park wouldn't be too crowded but Disney has scared me from visiting parks during the holidays