This can happen when you don't use the "pixel perfection" UI Scale. There is nothing I can do about it because of how the WoW Client render pixels on your monitor when a custom scaling is used. It's more or less the same problem on Windows 10 when you use the scaling feature, some elements on screen can get blurry.

PIxel Perfection is more or less 100% scaling, which render perfectly. Anything under or over 100% scaling will have some imperfection like displayed on your screenshot. And with too many different monitors, resolutions, aspect ratio, etc on the market, it is almost impossible to try to implement a fix for this issue with all ui scaling values available on WoW. There are too many variables to take into account.