Yeah, same here. Apparently this has been an issue for quite some time, but it seems there has been a bit of progress lately. You can launch steam with a forcedesktopscaling arg to scale the ui like this:

steam -forcedesktopscaling 1.25

Some people in the github issue thread have reported issues like crashing and an oversized overlay though. See: https://github.com/ValveSoftware/steam-for-linux/issues/5908