By default, Mendix generates a non-persistent structure based via the import mapping. It is possible to change this to persistent by editing the entities. Using persistent or non-persistent entities depends on your use case.
You can make it a persistent entity, however this is not recommended. This is why: in case you want to make a change to your persistent model, this is way harder to achieve when the import of an API is dependend on it. Therefore it's better to import in a non-persistent entity and copy the values to a persistent entity.
The concept of this software development concept is called ‘Loose coupling’ or ‘Tight coupling’. See also: https://www.techtarget.com/searchnetworking/definition/loose-coupling#:~:text=Loose%20coupling%20is%20an%20approach,one%20element%20has%20of%20another.