Installing HomeBrew `.app` on M1
Your last point of cask combining the files misses one key step. Someone else has to do the work to assemble and package that app. Homebrew doesn't do that assembly automagically.
I've installed mpv on three machines and your results are the same as I get - Intel Big Sur has a cask, M1 does not. Both get the command line binary.
Unless and until someone sends a pull request to add an M1 version of the cask for this package, you'll want to use a different tool or installation method.