Headcanon, but it's not bad headcanon. They never say that it was Elmer that killed Bugs' mother, but it can be inferred. Although, in the broader sense Bugs was referring to rabbits in general being victims of Elmer Fudd. Whether or not Bugs' mother was one of those direct victims of Elmer is not stated. We could then infer that it was a hunter that killed Bugs' mother. But, either way works.