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.
I believe it's Electron-based, which is another reason I've hesitated to try it out. I like Electron - from the developer's perspective, it's great! - but I do still try to avoid its resource impact until there's a compelling reason to take the hit.
That is one of the nitpicks that I missed, along with their downloads being excruciatingly slow when already connected to the service, for whatever reason (I may just be doing something wrong).