By definition a diorama is a model which shows a situation, such as an historical event or animals in their natural surroundings, in a way that looks real because the height, length and width of what is being shown are accurately represented in comparison with each other:
This means that it is possible to make a good diorama without using figures as long as you can compose it in such a way that the components of the diorama allows it to tell a story or show a situation.
In my opinion, every component of the diorama counts so if one is not done well, it can affect the overall impact of the diorama.