Okay, I just saw this same behavior and wanna post this FYI:
It looks like the Display Mode settings have reset to whatever resolution your Retina display is set to appear as. So, for example if you have a 5K display but have your screen size set to look like 2560x1440 (or 200% scaling), then WoW will now default to that in the fullscreen size settings and then upscale it so it’s all fuzzy.
The fix is to just go into your graphics settings (in WoW) and set the Display Mode screen size to whatever your actual resolution is, so for 5K it would be 5120x2880, for 4K it would be 3840x2160. Then, make sure that your Resolution Scale setting it set to 100% (which should match your resolution).
This should clear-up your fuzziness right away!
Disclaimer: Note that this is only a good idea for folks who were already running at 100% of their native resolution before the patch. If you try running 4K or 5K native resolution at 100% scale on an integrated GPU, it won’t be pretty.
Hope this helps!