Playa Del Carmen Area
Due to the Yucatan being the closest Mexican location to Europe there is a bit more tolerance to topless/nude activities. In the more populated areas like Cancun, Tulum and Playa del Carmen I would only employ a monkey-see, monkey-do philosophy towards exposing yourself. The three trips that I have taken to these places I witnessed toplessness of women, but nothing more dareing.
However, population thins out as you head south of Playa del Carmen with many ex-pat owned villas dotting the coast. I spent 3 weeks with my girlfriend (now wife) and another couple camping on beaches and occasionally staying in rented beach huts. Finding a beach to yourself was easy, so it was easy to lose the suits. As well, 2 of the 3 villas we stayed at had American owners who were completely fine with nudity. This was only 7 years ago.
edit note: spelling errors