The orange contour seems to be sufficiently high resolution. If you do not wish to see the underlying labelmap representation (that is always made up of rectangular voxels) then you can choose to view the closed surface representation (Segmentations module / Display / Advanced / Representation in 2D views: Closed surface). If that is still not smooth enough then you can reduce the spacing even further.
Note that in recent nightly builds, you can adjust voxel size in an existing segmentation (in Segment editor, click on the icon on the right side of Master volume selector). You still need to re-import the model but you don’t need to create a new segmentation each time you change the resolution.