I think if Zimmerman denounced his telegram as a British forgery he'd find a fair number of people willing to give the claim credence, as the telegram was a pretty poorly thought-out diplomatic move that seemed tailor-made to bring the US into the war. There would certainly be plenty of skeptics who would think that the Zimmerman was just too perfect a casus belli for the US.
That said, I agree with the others that by the time of the Zimmerman Telegram US entry into the war was a foregone conclusion.