It depends on which place u wish to travel, whether you want to travel to Kochi, Kannur, Wayanad, Trivandrum n so on.. If it's city life n beach then October, November, December as in other times u sweat like crazy because of humidity bt if u r a nature loving person ten go at the start or end of rainy season June to Oct..
My plan was like below : i spent 3 months in kerala in 2017 Munnay , thekkady , trivandrum ( varkala and kovalam beach along with puvvar backwaters) , and then head to kanyakumari apart from this i also visited Alleppy and athirapally falls
If you are choosing hill stations then any month would be fine. I went in May and weather was damn good. if you are choosing beach places then choose October to Feb.