The web version is mobile friendly and can be played right in your mobile browser. If you want to play offline, you can use the JoiPlay app or similar and the offline browser version.
Probably not. I feel like the available options for play on Android work fine. Any app I make would just be like a worse version of JoiPlay that only plays the one game. The only benefit would be that a dedicated app would be very slightly more convenient to install.
