David Oxenford's Broadcast Law blog is always informative, well written, and copyright-related. Recently, he discussed why, perhaps, "The Wind Done Gone" was a transformative adaptation of "Gone With The Wind", but the bold insertion of Star Trek characters into the Dr. Seuss story, "Oh, The Places You Will Go" was copyright infringement.
Oxenford does not delve into whether or not the JibJab political cartoons based on copyrighted songs were fair use or not. His analysis is thought provoking.
One wonders whether or not a mash up of news footage was inspired by the CME newsgroup advertisement How The World Advances with Brittany Lincicome (she drives a golf ball and it ends up at the feet of Sir Richard Branson on a large plane's staircase). Is inserting a golf ball or three into a news clip "fair use"?
Just because you make a funny version of a copyrighted work does not mean you are safe from a copyright infringement claim.
All the best,