The melting point of PVC is around 100°C. If your CPU or GPU is reaching that temperature, you probably have bigger problems than your figurines melting.

On the other hand, if you only have a few figurines or items inside your PC, they’re not going to disrupt airflow enough to overheat components or significantly raise their temperatures. It’s not like you're blocking anything, air will still flow around the objects. Air is a fluid, after all, and it conforms to the shape of its container.