My understanding is that it caused the price of corn to go way up, meaning the poor in Mexico found food far more expensive.

International markets and all that...

NAFTA and successive treaties forced Mexico to allow much cheaper corn from the USA to displace many smaller corn farms in Mexico, only larger farms could survive that and farming of corn is declining in Mexico. Also inexpensive processed food displaced traditional diet goods at the same time for the same reasons, probably contributing to obesity epidemic that only started in Mexico after those treaties.