Yup, most of the south cities are so fucking dirty. And it is just because of ignorance of people. North cities are the cleaner with the right exceptions, Tijuana and Cd. Juarez are one of the worst ones, but Chihuahua and Monterrey are in the first place of the cleanest cities of Mexico and Chihuahua is in the 9th place of the most secure cities of Latin America
I´ve been at cancun too and as Moose said, sucks. Mazatlan is ok. It depends in which side of the beach you are, public places are mostly dirty, while reserved places (for some hotels, restaurants, etc) are very clean and beauty.
BTW, dont move to mexico right now, it is 107 fahrenheit out there... omg too hot.