It's a general impression from years of reading newspapers. I'm not an expert, I haven't worked out the probabilities of what Mexico would be like without NAFTA, and there are dozens of other confounding factors such as corruption, competition from Asia, war on drugs, innovation-stifling industrial policy (see Carlos Slim's fortune) etc..

I was more focusing on the uncertainty created by the unclear messages from Washington, which does materially delay investment and confidence within Mexico, and makes it harder for the state to focus on defeating the cartels. Though I do believe that trade generally has a positive impact on economies as a whole.