Corn is fungible, like oil. Once it enters the stream of commerce, including the futures market, it’s not identifiable to any particular place.
There has long been resistance to American corn in Mexico, for all sorts of reasons. Part of it is the GMO argument, which is really just a trade barrier. Part of it is a belief in Mexico that American corn doesn’t have the nutritive qualities of Mexican corn. Part of it is the complaints of Mexican corn farmers that American corn is subsidized, which it is, and therefore unfair to them. Corn is basically an industrial commodity in the U.S.; an ingredient to further processed products, and it is in Mexico as well, but it’s more than that in Mexico. It’s kind of an institution with a lot of local variables that American corn cannot and does not replace. This will be good copy in Mexico, and popular, and it would have been just as popular without Trump.
It won’t sound too good in Iowa, of course, despite the fact that corn is really cheap nowadays anyway, due to massive overproduction, excessive insurance payouts for crop failure, and price supports, and would be no matter what Trump did or does.