Is make hay like sunshine is a idiom?

No, the idiom is actually "make hay while the sun shines". It means to do something when the conditions are right or optimal.

Example "I have enough money to buy an investment property, so I may as well make hay while the sun shines."

The idiom arises from the fact that hay is dried grasses, and in the past (and even today in some places) these grasses were/are sun dried, and the best time to do that, obviously, is when the sun is shining.