A simple answer is that if you plan to disable networking then security is a much lower concern as there is no reasonable attack vector. You would still be able to access your droplet via the console in the control panel but without networking, things like uploading content or installing software would be impossible. So, yes, a droplet without networking is very secure, but it's not very useful for most things.

A better option would be to install and configure a good firewall. If you choose, you can use a white-list configuration that only allows access from the IP address(es) you define and block everyone else on the internet. This may provide a better balance between security and usability.