In Nix, it's very easy to make a minimal mutt variant by removing the python input.

Yes, but these things shouldn't be happening. Default builds of packages should be kept more minimal.

I understand it's hard with Nix philosophy, and things are improving with different package outputs. For the record, python was pulled indirectly via the gnupg dependency I think.

It's a difference in philosophy. I think packages by default should be full-featured, and minimal variants can be created for those that want it.