I found kid3-cli
in our repo, it is not an extentension of cmus
but it supposedly can edit mp3 tags. I never used it, so I cannot vouch for you how good it is.
$ eopkg.bin info kid3-cli
kid3-cli package is not installed
Package found in Solus repository:
Name : kid3-cli, version: 3.9.5, release: 23
Summary : Audio tagger command-line-interface
Description : Kid3-cli offers a command-line-interface for Kid3, for a GUI use kid3-qt.
Licenses : GPL-2.0-or-later
Component : multimedia.audio
Dependencies : libstdc++ kid3-core qt5-base libgcc glibc readline
Distribution : Solus, Dist. Release: 1
Architecture : x86_64, Installed Size: 264.00 KB, Package Size: 75.00 KB
Reverse Dependencies: