US dollar is quite popular in Mexico and is widely accepted at local shops and malls. You will not face any problems whatsoever to directly use the US dollar coins or bills for purchases. They will be glad to accept USD. Big stores like Walmart, Sams Club, and Costco will also accept US bills but will return the change in Mexican currency only. Some people use the USD at Walmart to get a better exchange rate than currency exchange shops. Sometimes, Walmart and other big store's exchange rates are higher by about 5 cents per dollar.