I realize that mine have it also but since I don't mind little things as this (cause what's important to me is that it will not affect my workflow) then I unconsciously ignore.
Depends on where this came from, like if it was from DE thing all I can say is that fixing that part to be so smooth is something I don't know if it will be easy. UI/UX thing, in order for the design to be implemented, it needs to be coded. Coordinates etc in coding to create a perfect or smooth round corners isn't easy (well css have it already but since it wasn't part of the theme, I don't think simple CSS styling can solve it)
Im currently using Zorin 15.3 lite, it other words, its not just in 16 but also in 15.3. There common denominator is XFCE. Anyone knows what language used in creating XFCE?