Trade balance alone is meaningless. The US is a much wealthier country/nation/state than Mexico, of course if barriers to trade are lowered people will import more from the cheaper side. This helps Mexico's developing economy.
Helping a developing economy is a laudable goal sure, the political question is whether it is enough to be worth inflicting costs on your own voters. Some of said voters will say yes of course, generally those will be the ones benefiting from cheaper products/labour, and not those who have been undercut.
US/Mexico trade balance pre/post NAFTA? If you go by the numbers alone, it's pretty clear-cut. Unless you know better than the facts of course...