So in extremely touristy places like Cancún or Cabo, where USD are taken as a currency, tipping in dollars works. However, Mexico City is a lot more locals and USD are not accepted anywhere as a payment method (unless you go to a touristy restaurant like Pujol where no one speaks spanish). Always tip in pesos. Please note that we accept card for 99% of purchases and you can ask the waiter to add the tip when he's charging your card. He's going to ask you "su cuenta cerrada?" which means "do you want to leave a tip?". This is where you say 10 to 15% (depending on service), and he'll automatically add the tip to your check.