Does the shape of a handheld dynamic microphone's windscreen (i.e. a rounded top vs a flat top) have any effect on the sound, or is it just for aesthetics?
Aesthetics... and yes...
I could probably drum up some intelligent sounding reason for having one shape over another, but the shape is not going to determine the performance of the windscreen.
If something did change it's performance, it would be material, density, thickness. At least one of those aspects could be altered by shape I suppose.
Yes, it is just aesthetics; the sound of a handheld dynamic microphone’s windscreen does not depend on the shape. It is just an accessory. The sound depends on the distance to the mic.