An intresting theory but yet, it seems wrong. The first point is that of obvious. Kangashkan is a normal type while cubone is a ground type. No malnutririon could cause something so big. Another is that how do you explain the pokedex info: Cubone wears the skull of its dead mother. How in the world does cubone's skull look like the skull of a Kangashkan. Another point is that Cubone crys for his mother every full moon because it resembles it. It also states the mother dies directly after their birth. This hypothesis however, says this is a cause of malnutrition. This means that why should the mother die? All in all, an intresting theory but conclusively pointless.