The pixelation is due to the 3D widgets using linear sampling without mipmaps, which causes aliasing if the UI texture effective screen-space resolution is off by more than half the render resolution.
If your widgets are always at the same distance, you may be able to work around the issue by tuning the widget resolution so that one pixel on it is approximately one pixel in the final render.
Otherwise you will have to modify the widget component to generate mipmaps when it draws: