The fonttools varLib.instancer does all the things that varLib.mutator does plus more. The reason both exist is historical, with the former derived from the latter, but being a more general tool that doesn't only cut static instances but also partial ones.
I think this duality only serve to confuse users so I propose we officially deprecate and later on remove varLib.mutator module.
Alternatively we could keep both entry points but make mutator simply be an alias to instancer.
We need to double-check whether the latter is in fact a feature-superset of the former.