Sharknado is trying to cash in on the so-bad-it's-good bandwagon... but it kind of works.
You're supposed to be either outraged and amused by the existence of Sharknado, a now two film strong disaster/horror TV movie franchise built around the improbable weather event of the title. Amused because... well, one imagines that's pretty self-explanatory, outraged because it bears all the tiresome hallmarks of a cult-film wannabe -- i.e. movies that try to prefabricate the kind of midnight-movie popularity that usually has to come about organically. (See: Repo: The Genetic Opera, the latest in a long line of films trying way too hard to be "the next Rocky Horror.")
I'm probably meant to be in the "outraged" camp, or at least the annoyed one. Ever since the late, great Mystery Science Theater 3000 (and its offspring, RiffTrax and Cinematic Titanic) worked out how to turn the perennial Sunday-afternoon (or post-hangover, any day) hobby of talking back to bad movies on TV into mass-entertainment, fans (like myself) of actual "so bad they're good movies" -- i.e. films that are entertaining because they tried to be good and failed miserably -- have had to endure a deluge of films lazily put together on the pretext of being "bad on purpose." Why spend money or try hard when you can sell the exact opposite as the next The Room?
Admittedly, it can be difficult to distinguish sometimes: The Evil Dead movies all openly play up their makers' fondness for old-school rubber-monster schlock horror to make their own low-tech FX part of the gag, but they're also still clearly aiming for the films to be good in their own right. Birdemic has all the earmarks of a Tim & Eric sketch, but appears to actually be the sincere output of a... unique mind.
In any case, I understand the drive to lump the original Sharknado in with every other "riff-bait" throwaway that thinks pairing subpar effects and failed celebrities will turn them into a frathouse hit. But whatever the intentions in its inception, Sharknado actually had a certain level of authenticity to it. The purposefully ridiculous premise is exactly the sort of thing legitimate "exploitation" filmmakers of the drive-in/VHS ages would've seized on ("Y'know how sometimes tornadoes suck up fish and drop them on dry land? What if it was SHARKS!?"), for one thing. For another, the former-celebrity casting (Ian Ziering, Tara Reid) are exactly the sort of folks who tend to wind up in these things i.e. recognizable names without much real claim to memorability. It even has the half-hearted attempt at relevance that was the hallmark of "real" B-movies -- the initial Sharknado forms near a fishing boat engage in illegal shark-fin harvesting. Oh, and also nothing interesting really happens for the first forty minutes or so, which is frustrating but familiar if you've ever sat through, say, Monstroid waiting for Monstroid to show up.
(Full disclosure: I've worked a convention panel with one of the Syfy executives in charge of the Original Movies productions; and they stated that they're aiming for "fun" but not for irony or parody most of the time.)