If you want to start the process with npm start then you have to do that inside ~/MagicMirror
However if you want to use pm2 then I would advise following the instructions again: https://github.com/MichMich/MagicMirror/wiki/Auto-Starting-MagicMirror
pm2 seems to be working fine on your raspberry, so I would start here:
Make a MagicMirror start script. To use PM2 in combination with MagicMirror, we need to make a simple shell script. Preferable, we put this script outside the MagicMirror folder to make sure it won’t give us any issues if we want to upgrade the mirror.
cd ~
nano mm.sh
Add the following lines:
cd ~/MagicMirror
DISPLAY=:0 npm start
Save and close, using the commands CTRL-O and CTRL-X. Now make sure the shell script is executable bij performing the following command:
chmod +x mm.sh
You are now ready to the MagicMirror using this script using PM2.