What is the requirement for applying media.net ads account for the website?
Good content. That’s it.
What about traffic? Somewhere I read that 15-20 % of traffics should be from tier 1 countries like the USA, Cannada, etc. Is it correct?
Yes. 
I have 25k+ views monthly. I registered, but they rejected. They didn’t mention the specific reason for the rejection. So, I guessed about the traffic it should be from the Tier 1 countries.