Simulacrum usage

I’ve seen several books where a replica of an object, through supernatural or scientific means, reflected its state onto the original object. These replicas were called simulacra, and I’m wondering if this is an accepted usage of the word.

The basic meaning of "simulacrum" is "image" (sometimes with the connotation "mere image").

The specific concept that you describe, of "simulacrum" being used to refer to some instrument of sympathetic magic, is not a usage that I can find in any dictionary. Something not being in the dictionary doesn't exactly mean that it is not "accepted" usage, although it indicates that it's probably not a very common usage.

Works of speculative fiction often use words in specialized ways rather than according to their general usage. I'm not sure what the history of this usage is. The earliest example that I know of is in the novel The Magician's Apprentice by Tom McGowen (1987), where the term "simulacrum" is used to refer to a wax statue made in a person's shape and containing a bit of that person's blood and hair that supposedly can affect the state of the person. I don't have an exact quote, but you can see a summary of the work that covers this point here: "The Magician’s Apprentice", Jestress's Forgotten Books and Stories (blog by Tracy Christenson).