Almost anytime is good to visit the Mayan Riviera, but for me the best season to visit the Mayan Riviera is February through May, excluding the Easter week. In the period May/August it can get extremely hot. December and January may present a few chill weeks, and X-mas and New Year is too crowded. Then, August through November are the months with the highest probability of hurricanes (peak is August/September), and it rains almost everyday (though there is also sunshine at least some 4-5 hours a day and you can enjoy the beach).
Peak seasons are X-Mas/New Year and Easter for Mexicans, the December/February cold months for the Canadians, August for the Europeans and January/February for the South Americans. Mexican schools close in July/August for summer holidays, so you will see more Mexican families these two months.
If you are a bargain hunter then your months are October and November. Occupation rates reaches their lows, and most hotels and cabanas offer incredible low prices (up to 70% less than in peak season).