I was having spotty results until I cleared out the QuickLook thumbnail cache. The location varies, but it'll be under /private/var/folders/.../com.apple.QuickLook.thumbnailcache. I moved the 300+ mb thumbnails.data file to my desktop and rebooted. After that, I still had to open Launchpad and drag the corrupt icons to fix them (but at least it worked).

Alas... doesn't work for me neither.

FYI didn’t work for me either, I have had the issue noted of an icon being blurry for a week or two then returning to normal.

I get this a lot, at the moment Chrome looks like a blown up 16x16 icon. I have tried clearing caches, fixing permissions, same results. It is very bizarre.

Sadly, this didn't help me; not even with my fuzzy Marked.app icon. ;)

Hmm.... doesn't help with fuzzy custom folder icons; I have to restart.