I think threadless stems look great on bikes with threadless steerers and 31.8mm bars.
The stem's diameter looks appropriate to me in this situation because everything is equally larger than than traditional sizing.

In general, I dont find the type of stem to be good or bad looking.
I find specific stems to be good or bad looking. For example, Nitto Technomic stems are about the least attractive stem I can think if right now.
I also dislike stems- threaded and threadless- with large visible welds.
A simple black or chrome threadless stem with small/hidden welds looks quite nice. To me, a stem should simply not attract attention.