As a non Budgie user, I am not best suited to answer this question.
However, I guess as mutter43 was a dependency of budgie-desktop so it must have been installed in Budgie installations at some point.
It is not a dependency of budgie-desktop anymore but magpie-wm is, as can be seen here:
https://archlinux.org/packages/extra/x86_64/budgie-desktop/
At some point of upgrading budgie-desktop, did you get any message about conflicting packages, that mutter43 was going to be replaced by mutter?
Edit:
If you want you could look into your /var/log/pacman.log and search for mutter43.