This app was mentioned in 9 comments, with an average of 1.44 upvotes
Taggr is the only one I know of. Though for mass (auto) tagging, I would definitely use a computer instead because even when you take the time your spend moving your library to/from your computer into account, you'll save much time (for large libraries that is).
Taggr works quite well and is Material Design (I only needed to use it once right when it was released, and since factory reset and did not reinstall it) https://play.google.com/store/apps/details?id=id.ridsatrio.taggr
Yes. You need a tag editor. I use Taggr.
Alternatively, you can use the app Taggr to download album art and edit artists/album names. It looks and works great, but it may take a while to download all of your missing album art.
Download from Google Play Store