Senor Calderon, no wall can be stupid.

If Presidente Calderon wants to get Trump elected, then he should continue to attack Trump. From the story:

"Mexican people, we are not going to pay any single cent for such a stupid wall! And it's going to be completely useless," Calderon said.

"The first loser of such a policy would be the United States," he said. "If this guy pretends that closing the borders to anywhere either for trade (or) for people is going to provide prosperity to the United States, he is completely crazy."

 
Firstly, Senor Calderon, no wall can be stupid. While a wall may be poorly placed for a number of reasons, they cannot, in general, think.  You are projecting your ideas of Trump onto a proposed inanimate object.  Secondly, while you are talking to Mexicans for the purposes of local political objectives, what you are doing is in fact stoking up and justifying the rhetoric of Trumpian Logic which his voters find appealing.   Anytime someone attacks Trump's emotional ideas - emotional because they can't possibly be based in reason - you are attacking the emotional voters that are pro-Trump.  Even if you laid out a coherent argument to the effect of nullifying the wall. they would act irrationally and vote that he double his \$8billion US wall into \$16billion US.

So any reason will be met with antagonism from an angry and unhappy mob. In fact, by attacking him, you are legitimizing his positions and making his voters believe that Trump might be onto something despite its' total lack of coherence. Or cost benefit analysis. Or even justification. 



In other words, if foreigners want to elect Trump - go ahead and stick your neck in the noose and criticize him publicly. Even Obama falls short of directly criticizing Trump BECAUSE he understands the moods of these voters.