No, not necessarily. Think of it this way: if you are just browsing the internet, your computer is not using 100% CPU either, right? But still everything works flawlessly. It’s just that there is nothing more to do so the device doesn’t need to work harder. To give you an idea, I am training a model on a single GPU and it is going steady at around 60% CUDA usage. That is fine. In your case, you will likely see more fluctuations because it is a multi-GPU set-up in DDP where GPUs will have to wait for each other from time to time. That is normal.
As long as the performance is what you would expect, you are good to go.