If I have been in US for Jan to April 2018, do I mention I was covered under MCC for whole year?

If you are a US citizen and moved abroad in 2018, you would put that you had MCC for the months that you actually had health insurance and then check the exemption that you have been out of the US for 330 days or more (so from April 2018 to April 2019), if this is your situation.