We haven't been to the Cancun resort yet, but we just got back from the Puerto Vallarta trip, and we did not need pesos. In the tourist towns, most everyone takes dollars, though you will get change in pesos. Plus, most places you go probably take credit cards, which will be billed in pesos at the prevailing exchange rate.
And at Vallarta Palace, in case we did need pesos, there was an ATM machine in the resort that dispensed pesos. I don't know if Cancun has one, but I would think they would. In any case, you can use any ATM machine in the country, and it will of course dispense pesos on demand. So, I wouldn't bring pesos. If you end up wanting or needing them, it's easy to get them.