Thanks for starting, at least editing possibilities in Plexamp are highly desired.
Moods on artist and album level makes no sense indeed. Besides that, I can’t relate / recognize a lot of the imported Moods to what it supposed to describe.
I doubt if it would work great if Track Moods are used (because the one who describes the Track Mood could have another opinion than I have, it is more or less subjective - some tracks which make me happy, are to the ears of my wife sad), but it would definitely improve Mood Radio if that’s used instead of using Artist / Album level Moods.
I didn’t want to wait for Plexamp using Mood Radio based on Track Moods.
I emptied all Track Moods and created a list of around 15 Moods to add to Track level: (Sad, Happy, Melancholic etc.).
And for some Moods I stretched the ‘concept’ Mood a bit: (Dinner, Friends, Sunday Morning, Late Night, Dancing etc.).
This way I can quickly select Tracks in the Mood Sunday Morning with genre e.g. Classic instead of making playlists.
But it is tedious and time-consuming.
Listening in Plexamp on my mobile or iPad and editing the Track on the desktop or MacBook😩
Which is why I created a feature request for that to resolve that bane (can’t be that hard, for a developer 
):
I started in Plexamp with creating playlists, but I discovered I could find my way with Track Moods. Those playlists and the ones I imported from Roon, Tidal, Spotify I wanted to edit in bulk to add to a Mood. But that is apparently not possible.
