Merchants will HAPPILY take dollars at a terrible exchange rate.
(They don't post the exchange rate, they post price in dollars, price in pesos, and assume you won't do the math.)
That being said, if you aren't spending much money, it may not be worth the effort to get the right currency.
(On our last trip to Mexico, the last day I just spent dollars, rather than trying to "come out even" getting pesos from the ATM.)