The short answer is "yes". The Plex app for Apple TV does require linking with a Plex account in order to be able to use the app.
Why use an account?
Signing in with a free Plex account can help your apps find and connect to your Plex Media Server without having to worry about all kinds of complicated network things. As just one example, it's useful when your Apple TV and Plex Media Server are on different subnets, which is a situation in which automatic discovery can fail.
Signing in to your Plex account has multiple benefits, including:
- Help Plex apps locate and connect with your Plex Media Server
- Many Plex apps (such as PlayStation, Smart TVs, and Xbox) will only work when both the app and server are signed in
- Let your server get a certificate to allow secure connections
- Make use of Plex Pass features if you're a subscriber
- Allows you to view your Watch Later and Recommended queues
- Access your Server when away from home (if you've successfully enabled Remote Access)
- Access shared libraries from family or friends
Related Page: Sign in to Your Plex Account
Related Page: Why can't the Plex app find or connect to the server?