While Mexico's native currency is the Peso, most places you go (including your Cancun hotel) accept the US dollar as currency. However, like most Caribbean countries, accepting the US Dollar without a currency conversion will mean you will be on the short-end of the exchange stick, sometimes as much as 25% in Cancun business' favor.
It's best to exchange your currency for the Peso, ensuring you get the most purchasing value out of your money. Consider hitting a Cancun bank or 'Casa de Cambio' and be sure to check the exchange rates. Your exchange rate will change daily and also vary from place to place, with some of the worst deals coming from your Cancun hotel.