Does it work in the login screen?
Have you followed the instructions that can be found in the official support pages to install Plasma? Installing a different DE in Pop (or any other distro that comes with a DE preinstalled) isn’t so simple. You may want to test if the problem persists if you login with a brand new user, if not then it means that something in ~/.config is probably broken.
Hey, mmstick! Thank you for all the work on Pop!_OS and moderation as well!
Because it changes drastically how the extension works: it moves the extension from the battery menu to a separate panel in the top bar, so I though there was little to no chance for it to be merged. Also, I could not test it myself with Pop since I’m not using it right now.
But if you think it’d be useful, I’d gladly submit a pull request.