I was having the same issue. When I fiddled with the display settings it would be fixed…but only temporarily. I found this response to a similar question and it fixed it…as far as I can tell. It was definitely frustrating as I JUST got a new computer last week and wasn’t having those issues then.
https://us.forums.blizzard.com/en/wow/t/pixelated-particles/898593/2
I hope this helps.