I do agree, it does not make lots of sense to reach the back-end. In the end, it is a prototype rather than the functioning product.
Remember the main focus is evaluating the experience and the interaction of the product, not the content itself.
If the content plays a major role in the experience of using your product, you have a couple of options:
You can ask your testers to assume that the data is whatever is relevant to their context at the time of the test.
You can produce static content that simulates the real scenarios, which should be sufficient.
From the other hand, if the prototype you are developing is capable of taking the user's input and processing as the real product would, it will increase the accuracy of the user tests, but should not come at a high cost of resources, otherwise, it will defeat the purpose of prototyping.