My mullvad installation on Windows has 258MB but memory footprint is low. I find 5 entries in the task manager with a total of 14.6MB with active connection.
Maybe not Electron, then. Perhaps I'm confusing it with ExpressVPN's first-party app, which definitely was Electron when I tried them a few years back.
throwanem|3 years ago
Foxboron|3 years ago
https://github.com/mullvad/mullvadvpn-app