The quality of text labels of markups fiducials is not optimal, the text is not anti-aliased.

The problem is that the text is rendered using polygonal data (vtkVectorText, in vtkOrientedPolygonalHandleRepresentation3D) and by default there is no anti-aliasing of poldata in render windows.

A solution would be to use a different representation of vtkSeedWidget in slice views, where the text of each handle is rendered using vtkTextActor (the same way as text is rendered in ruler and other widgets).