That’s probably because your files have embedded cover art with relatively poor resolution.
Or there are external cover art files in the folders (some of them might be hidden, see How do I remove an incorrect tv show poster )
Which version of the cover art Plex is prefering depends on the order of the lines under
Settings - Server - Agents - Albums - Premium Music
If ‘Local Media Assets’ is at the top, then embedded /sidecar art is preferred.
Unfortunately, there is no way to tell Plex to ‘prefer my embedded meta tags, but use the cover art from Gracenote’.
