At this point, we don’t know much about Mendoza at short. He was there in January instructional camp and the little bit of spring training, but minor league camp never started. So he has yet to play any meaningful time there in real games.
DeJong proved the move from third to short is possible, even if quite uncommon. But my guess is that this is broadening Mendoza as a utility player, not to position him as a serious candidate to replace DeJong one day.