Given the number of companies who'll provide you with a fully managed Postgres installation there's no reason not start out on it for the MVP - as a user its no harder than MySQL. Even as a sysadmin its not significantly harder, just different to work with to MySQL.
I would say that if you are just starting out and you already know MySQL and you are proving out your MVP, MySQL is still relevant.
Once you have a stable product and you need a better database, PostgresSQL is a good move up.