No, the Kubfu Mustard gives you cannot be shiny. It is impossible, as the developers intended it to be “shiny-locked,” a term in the Pokémon community referring to how certain Pokémon, especially Legendary and Mythical Pokémon, can never be shiny. However, there is a shiny variant, although to get one, you would have to cheat, and that is illegal (Galar police, get this man!).