Well, not necessarily true given how so many comedies these days are the dark kind as well.
Again, The Room is considered a dark comedy by it's creator but since that film has a cult fanbase (possibly almost as high as Twilight), made numerous "Worst Films of All Time" lists and was considered to be unintentionally amusing, it was riffed here.
They're also NOT going to riff intentionally bad movies like anything by The Asylum, intentionally cheesy SyFy Original movies or even Birdemic 2/Hobgoblins 2.
For the record, Casablanca has never been considered a comedy (it is however considered to be a pleasant classic drama) and was just done on a dare between the crew to see if they could even parody one of their favorite movies of all time.
Ocean's 11 and Back to the Future both have comedic elements but are both Crime and SciFi respectively. In other words, they were "Part-comedy, part-[insert genre]" not a full-based comedy like something Woody Allen, Judd Apatow, Kevin Smith or Frank Capra would make. This is largely due to the RiffTrax crew agreeing with Joel, Trace & the Cinematic Titanic crew that making fun of other comedy would just make one look like a pompous ass (their words not mine) due to it's subjective nature and basically abusing the premise in exchange for mean-spirited laughs.
For instance, let's say you're not a fan of a popular sitcom and you decided to mock it right there and then. Who's the bigger idiot: You or the people on the small screen? My brother and I used to do this with newer episodes of Sesame Street and it was no fun at all...
For the record, I do agree that if it's something as goofy as Santa Claus Conquers the Martians or a low budget "comedy" like Manos that it can work because there's enough "WTF?" type moments and nonsense to where you have to wonder what the filmmakers were thinking at the time as well as other elements that keep within the original riffing premise.
Making fun of something as tasteless as Epic Movie isn't going to win any favors since you can't do much with a poor parody except remark on how insipid it truly is, which again, is arguably not funny, it's now turning into a rant session such as That Guy With The Glasses.
As for why they parodied a short film of The Three Stooges, they were mainly doing what they did with Ed Wood's Plan 9 From Outer Space, Reefer Madness and the various Godzilla/Gamera films in just appreciating the campiness of it all while also having fun with the inevitable flaws that are bound to occur in such movies.