I looked at this for myself earlier, and found the following problems:
the iTunes tag is only applied (by iTunes) to mp4 files not mp3's
you can set the tag up like so
the tag syntax is not in the form of 'Explicit' or 'Clean'
0 or null = No Rating,
1 = Explicit,
2 = Clean
you could use those values to set up a highlighting rule for
Content Rating | is | 1
but if you want to actually have 'Explicit' or 'Clean' show up as words in a column, you would also need to use a virtual tag
Content Advisory: $IsNull(<Content Rating>,,$If(<Content Rating>=0,,$If(<Content Rating>=1,"Explicit","Clean")))
I found it easier to just add 'Explicit' into my keywords and use a highlighting rule.