I believe OP is talking about currency exchange onboard for using in port in Mexico, not for spending while on the ship.
ATMs almost always have the best exchange rate if your bank doesn’t charge a currency conversion fee, but finding the right combination of menu items in the ATM to find get pesos charged in pesos without a dynamic conversion to dollars can be a bit of a challenge. Most ATMs really want to “help” you by billing to your bank in USD (and charging a 3% fee).I’ve always read thstvATMs are the best and cheapest to exchange money. Our Bank doesn’t charge a fee. But we go to Mexico every year. You really can get by with US dollars as long as you know the exchange rate