It is really hard to tell actually. Unlike the Greeks and the Romans, the Egyptians didn't really have any historians who wrote down texts for us. What we have are mostly inscriptions and the like. We think they did. The dynasty is part of what is known as the Late Period and while the rulers of that period are rather wide-ranging, they seem to have been acknowledge as legitimate rulers of Egypt. There were no other rulers of Egypt during those times and they kept the state together.

However, we also know that Alexander the Great was greeted as a liberator when he came to Egypt during the 4th century BCE, which suggests that while the Kushites were legitimate rulers, they were unwanted such. It is quite a stretch to compare that to the Nazis and Vichy France though. I mean, I can see where you get the idea and why it seems like a fair comparison, but there is an important difference. The Nazis in Vichy France occupied the state and immobilized the French ruler(s).

The Kushites on the other hand didn't actually do anything like that, there wasn't a rightful ruler to put on the Egyptian throne. They became rulers after the turmoil that was the Third Intermediate Period and they united the Egyptian state (which is an important aspect of Egyptian history). Thus they were the only legitimate rulers of the state at the time. On the other hand, one could say that the only difference between the Late Period and the Third Intermediate Period is that Egypt was unified, because the rulers weren't just Kushite, there were also Persians ruling Egypt in later dynasties.