Many years ago when I was working the SoCal Renaissance Faire, I asked Adam Long (the founder of the Reduced Shakespeare Company) why he and his cohorts didn't parody any of Shakespeare's comedies. In my naivete, I thought "Taming of the Shrew" would be perfect for them. He answered me with a piece of wisdom I've often pondered:
"Comedies aren't funny."
He was right then and you guys are right now. It's useless to even attempt a parody of a comedy. The more solemn and puffed-up a film is, the better it is for Rifftrax. Case in point - the masterpiece that is the RT version of the Matrix trilogy. It's the only way I can watch the 2nd and 3rd parts of that, because the originals are so utterly brain-destroying.