There is no inter-city bus service available from Cochin airport, so you won't get direct buses from Cochin airport to Munnar. There are frequent buses to Munnar from Ernakulam bus stand. You can catch a city bus to Ernakulam from Cochin airport, from where you need to take another bus to reach Munnar. Cochin airport to Ernakulam is around 30 KM and it takes close to 1 hour by bus.
Aluva can be a better alternative to Ernakulam. You can get buses to Munnar from Aluva without entering the Ernakulam city. It is the nearest bus stand to Cochin airport from where you can get a direct bus to Munnar. There are frequent city buses to take you to Aluva from Cochin airport - which is just around 12 KM away. Aluva to Munnar distance is just about 115 KM. There are lots of KSRTC buses to Munnar from Aluva bus stand. You need not have to purchase tickets in advance. Just go to the bus stand and board your bus to Munnar. It takes around 3.5 hours to arrive at Munnar from Aluva by bus.