A US traveller in Playa del Carmen does the maths at 20% out of habit, then realises the waiter looked surprised — pleasantly, not confused. 15% would already have been a generous, completely normal tip.
10% is the baseline for acceptable service at most Mexican restaurants, with 15% recognised as a genuinely generous tip for good service. 20%, the US default, is well above what is expected and can come across as a tourist overcorrecting rather than a cultural requirement.
Many Mexican servers are paid partly through tips that are pooled and split at the end of a shift, and cash is far easier for a restaurant to distribute fairly and immediately than a tip added electronically to a card payment, which may be taxed or delayed before it reaches staff. Carrying small pesos for this purpose is genuinely useful.