I am not sure about legality of being naked on a beach in Mexico. But I have never heard of anyone be fined or arrested for being nude on Desire beach.
As Mexican nudists for more than 20 years, I have dedicated myself to researching the laws of my country about the legality or not of nudism. Each state has its own laws and they vary very little in this regard. Fortunately, the crime of "moral offenses", the description of which was generally very vague and open to abuse by the authorities, was long ago eliminated.
The crime of "obscene exhibitions" persists, of course. This consists of performing acts of a sexual nature in front of other people without their consent. It is aggravated if they are carried out in front of minors.
In such a way that being naked on a Mexican beach is not illegal at all.
There is also what we call "uses and customs"; Under this concept, nudism is acceptable in Puerto Morelos and Tulum; while topless is accepted throughout the state of Quintana Roo.