How to unrate multiple album ratings at the same time?
I was looking to do the same thing the OP requested here. This was my solution:
Open the Music app (formerly iTunes in macOS 10.14 and earlier) and do one of the following:

create a new smart playlist with the criteria that album rating does not equal zero stars (the star to the left of the first star) then open up that smart playlist and select all the songs therein
sort tracks by album rating and select all the songs that have a non-zero album rating.

Next, open up Script Editor.app and paste in the following script:
tell application &quot;Music&quot;
    repeat with theSong in selection
        set album rating of theSong to 0
    end repeat
end tell

You may need to run it more than once as it may fail as the selection changes while songs are being removed from the smart playlist. After a successful execution, all manual album ratings will be removed. Some albums may still have grayed out ratings: these are from the song ratings themselves and aren't actually album ratings.