This is from the main installation page at https://github.com/MichMich/MagicMirror#manual-installation
Raspberry Pi Support
Electron, the app wrapper around MagicMirror², only supports the Raspberry Pi 2 & 3. The Raspberry Pi 1 is currently not supported. If you want to run this on a Raspberry Pi 1, use the server only feature and setup a fullscreen browser yourself.
Server Only:
In some cases, you want to start the application without an actual app window. In this case, execute the following command from the MagicMirror folder: node serveronly. This will start the server, after which you can open the application in your browser of choice.
I hope this helps