I think mutation testing really shines in code bases that are already heavily tested, because it let's you discover test cases that you don't actually need. Tests are a burden since you have to adapt them when you change the behavior. With mutation testing you can prune your test code by identifying tests that test very similar behavior.
Would that be done based on which (different) tests killed a particular mutant?
Often an integration test would catch the multiple mutations also being caught by (different) unit tests.
I assume that you mean that if a certain (broader) test kills the same mutants as X unit tests, those X tests are not really necessary?
I've looked into https://github.com/boxed/mutmut and https://github.com/sixty-north/cosmic-ray for Python project, and there it is only important that a mutant gets killed, but not how often and by which tests (therefore you can use `pytest -x` to continue with the next mutation after the first test failed due to it).
I usually use coverage as a tool to remind me of pieces of code I intended but forgot to test [1], so setting it to an arbitrary percentage is not that useful, in my opinion. If 100% is not feasible usually as well, that makes Mutation Testing in general not worth the effort.
Again, take my opinion with a grain of salt.
[1] https://vincenttunru.com/100-percent-coverage