Maybe see the discussion on:
and
In short: You can unset some variables you don’t want to write or mark variables as not to be changed / overwritten. But in general Picard is designed to tag your files with data from MusicBrainz.org and is not very well suited for updating only a single tag. And as I have written in the other tags I would advise you to keep the MusicBrainz IDs.
I think you should learn a bit on how to tag with Picard. Have a look at http://picard.musicbrainz.org/docs/guide/ , this shows you how to do the usual Cluster + Lookup search, which often provides the best results. If Picard loaded the wrong edition of your album you can right click on the album and choose the correct one in “Other versions…”. Or you search manually in the database, see Loading releases from musicbrainz.org into Picard
And if all fails please add your data to MusicBrainz to improve it for everyone.
I am neither using Picard-Last.fm.ng nor MP3Tag, but Tag Field Mappings – Mp3tag Documentation su
ggests MP3Tag does not support the mood tag for all file formats. What format are you tagging? And in case of MP3 it is only supported for ID3 v2.4, so make sure to configure Picard accordingly in Options > Tags