Make Mexico a state. Good Lord, you are serious. And here I thought the depths of your stupidity had been reached.
Putting aside the fact that most Mexicans would not appreciate being conquered, making Mexico a state would make the occupations in the Middle East look easy.
Mexico's problems wouldn't suddenly disappear, they'd just become the United States' problems.
How do you fix the standard of living? Government programs? Well, that money has to come from somewhere, and that probably will mean the current US will subsidize them heavily. So, good luck with that.
The cartels won't go away instantly, either. The federal government would have to deal with them (again, money and effort). Never mind that those same cartels, along with other gangs and mafias already operate within the USA currently. So how would the USA invading Mexico do anything but spread the USA so thin that the Cartels would have more power?
Also, dealing with the cartels/ occupying Mexico means you’d either have to massively increase the size of the US military in terms of materials and soldiers in uniform (good luck with that) or massively shift support away from the USA’s shadow-empire overseas. Which is unthinkable you, and every neo-con and other politician in the pockets of the military industrial complex.
The illegal immigrant problem, even, wouldn't disappear, it'd just change. There still would not be as good of economic opportunities in Mexico, but now all the Mexicans looking for better lives could legally migrate north. Congrats, you’ve just made the problem 50-times worse and given the Democrats even more voters to appeal to.