Not sure if this have been requested previously, but it would be an interesting addition to the app. Most of my library is ALAC and I manage it using both iTunes and Mp3Tag to add other tag fields.
ITUNESADVISORY with a value of "1" indicates "Explicit", while "2" denotes "Clean". There's also a "0" value, which is just the same as if there was no tag attached to the file.
Hopefully this is something that can be implemented as it's already shown in all music (streaming) apps, and it'd be a cool feature for Poweramp as well.