The weather in Chiapas [1] varies considerably from region to region, especially those at different altitudes. Most travelers prefer the dry season (roughly November to May), when the state’s famously blue rivers are clear and rural roads more passable. (It can be quite cold then, however, especially in the highlands.)
If you want to beat the crowds, avoid the peak travel periods around Christmas and New Year’s, Semana Santa (the week before Easter), and July and August. That said, it can be interesting to visit during local holidays, like December 12 (saint’s day for the Virgin of Guadalupe) and Carnaval (end of February).
Links:
[1] http://www.moon.com/destinations/chiapas