Since too hot during may ... You can plan during early days of March ... if its only for Munnar you would go during may :) Enjoy your trip and do share your experiences :)
May will be core summer time in kerela near to monsoon. I dont think it would be a great idea to visit kerela in May. You can plan for feb this month or at the end of year like november to January. Thanks.
May is a nice time to visit with the temperatures falling down and with the beginning of rains. I've personally visited Kerala in May and it was a good experience overall.