I'm always puzzled by folks who approach Michael Moore as a standard documentarian. He's not. He mixes his documentary-style film-making with entertainment. He always has. He makes no bones about it. I haven't seen Sicko, but I doubt he doesn't use irony, absurdity and humor to make most of his points.
There are lots of humorists on the left, whether in comedy, film, documentaries, literature, etc. You could name dozens and dozens. But why is it the domain of the left wing?
The right has the Moral Outrage thing nailed, but I'm always amazed that it can't seem to get its arms around the idea that humor is a great source for discourse. I'm sure there are plenty of funny Republicans out there.
I'm politically non-denominational when it comes to humor, but the only ones I can think of that are any good on the right are Dennis Miller (who is hilarious) and P.J. O'Rourke (who I don't think is as prolific as he used to be).
So if folks are upset by the fact that Moore is a left-winger mixing entertainment with his facts, they should foster some creativity in their ranks to approach the same. No need to take ourselves so seriously.