So, what if Carranza's government decides to act on Zimmerman's telegram not by accepting it, but instead by assembling their best writers and psychologists to craft the most insulting reply they can think of, in the vein of the Reply of the Zaporozhian Cossacks. The goal is to take advantage of the Kaiser's known irrationality and temper by insulting him and his government into declaring war or ordering submarine attack on Mexico. In doing so, Mexico may be able to receive more foreign aid in the ongoing civil war as an "ally" of the Entente. Could this plan work at all?
EDIT: Now that I think about it and after some time in Wikipedia, this move could be politically beneficial for Carranza by countering unwanted American influence with more palatable British and French influence.
Further, Carranza could use the "unjust" war against Mexico as a way to remove politically unreliable units by having them ground down in France.