OK to power off Apple TV (4th gen) at the mains when not in use?
Yes - you should be fine. The Apple TV runs iOS which has several technologies to help maintain system reliability when it shuts off abruptly.


Apps are sandboxed and easily deleted and re-installed from the App Store if they should get corrupted data or settings.
The OS is easily wiped and restored from the network.
Changes and writes to storage are low compared to computers. The filesystem is journaled and most of the time it reads data and pushes pikes to the sound and video attached as opposed to doing large calculations or maintaining data files where corruption is a problem.


You will save yourself the time to reinstall things if you put it in sleep first (that flushes all writes to storage) but I say you could treat it however you wish and only change to sleep / shut down if you find you are having to maintain it / reinstall apps. Probably, you won't need to take any care of it.

The TV is designed to be extremely low power usage in sleep mode, and you give up several functions like rapid wake from sleep, bonjour sleep proxy for other Apple devices like wake for back to my Mac and HomeKit automation since the TV works as a "hub" for those activities even when it sleeps. If you don't notice problems and re-cabling the TV to always have mains power is inconvenient - just keep doing what you habitually do with your entertainment power.