Not a chance. There are far more preposterous finishers that have been used by some successful superstars (main examples: The Rock, Mick Foley, other: Umaga, Scotty Too Hotty, Crush). A finisher isn't about looking devastating, it's about being something with a little flair -- it's about the crowd identifying with you. WWE isn't about being believable, it's about suspending your belief. I don't need to think that one of the talents is hurt, I just have to react to the move; whether I laugh, gasp, scream, shout, jump, whatever, I just need to react. Santino's move -- as simple as it is -- works.
Have you ever just paid attention to the crowd when Santino pulls out his green Cobra sleeve? The crowd goes WILD! His moment at the Royal Rumble with Mick Foley was one of the highlights of the night -- that says to me that if it wasn't a proficient finisher, then the crowd wouldn't care about it, but they clearly do. Being a US Champion doesn't change that either. Santino Marella has done more for the United States Championship than Zack Ryder and Jack Swagger did, and neither of them uses something as simple as the Cobra as their finisher. The thing is, neither of them is anywhere near as good as playing to the crowd as Santino, neither of those guys is anywhere near as good of an entertainer.
I don't need to justify the Cobra to myself, because it's literally what wrestling is all about. If I wanted hard hitting moves, I'd watch ROH (speaking of which, they have a far greater problem with finishers) or just plain MMA. Santino's fine.