Well, having travelled all over the Mayan Riviera in almost every month; the one thing I can say is that I have been there for cool weather several times. Everytime a Norte blows in it is down right chilly, have experienced them in November/December and the worst was in Late January into early February.
I do not like Cancun (way too commercialized and americanized for my tastes) and would not go back there again except to catch a flight. Each to their own I guess.
It has been very rainy the past few months and friends of ours who live there claim that the weather has never been more unpredictable than it is now.
We travel in September often and really love Mexico at that time; cheaper prices, less crowded and its warm....something that January/February cannot always offer. We were in Puerto Morelos in February (I think 2008) one year and the locals were all wearing touques, scarves and warm coats. I think it was about 10 celcius if I remember correctly.
No guarantees, no matter when you choose to go. Enjoy the experience no matter when you choose to go.