The ADO bus tickets can be purchased with a credit card (at least at the ADO desk inside the terminal). That said, I'm always comfortable having at least a little local currency in my pocket...in this case, for the ferry (although I suspect they will take a credit card, too).
I figure every time you pay for anything in foreign currency, you're getting a bad exchange rate. Sooner or later you're gonna want pesos, for the bus, the ferry or the taxi to your hotel, whatever.
There are a few ATMs in the CUN airport (departure terminal), they're pretty much right on the way as you're dragging your bag to the bus.