You don’t need to clone the repo to install it. There’s a compiled version of it under Releases.
You can get the latest here: https://github.com/cweirup/JoplinSafariWebClipper/releases/download/v.0.1.3/Joplin.Clipper.zip
Just unzip, move the executable to the Applications folder, and run the app once. You’ll be prompted to enable it in Safari Extensions, then you should be good to go.