Yes, in this case, void* is kind of smelly. If the intent of your function is to receive a const struct MyCustomData*, then that should be the type of the argument. If you later need to handle a const struct MyOtherCustomData*, you can add an overload that takes that argument. Or use a template as others pointed out. Use the type system to help you, so you're warned if you try to pass the method const struct BadCustomData* by accident.

If you truly don't know what the underlying structure of the "blob" of data is, sure, go ahead and use void* and explicitly convert the pointer type when you know what it is, but at least add a comment that you're entering the danger zone.