Here is a Weatherspark comparison with both Brownsville and Ciudad Victoria roughly in lowlands:
https://weatherspark.com/compare/y/6...nd-Brownsville
Looks like muggy humidity almost 100% of the time in Brownsville and average high temperatures over 90 in Ciudad Victoria, listed as Sweltering. By the time you get closer to Mexico City and at elevation it will become more comfortable, but you have a lot of hot/humid cycling to get there.
I knew a woman that lived in Brownsville and she said it was hot with lots of humidity.
Sucked even more than the mid atlantic summers.