I started a PR here: https://github.com/microsoft/pxt-common-packages/pull/934 but it’s far from complete. It would be useful to get a few data points on what you see when the joystick is idle with the program attached there. I’m thinking dead zone of -50 to 50.
If you want to do something right now, you can use this program, though it will fail in the simulator:
I guess you could check if the config value is 0, and don’t do the analog read then.