I played the downloadable Windows version. In my experience HTML5 versions have some sort of issues most of the time. Maybe the jumpiness of the mouse look (i.e. very high sensitivity) depends on the screen resolution. I have a 2560x1440 screen here and I played on fullscreen. I'm not sure how Godot handles mouse input in regards to viewport size. But in general, having a sensitivity slider would allow people with similar issues to work around such problems.
If you'll pursue this concept in the future, I'm looking forward to see what you'll do with it.