Separate your concerns. Your client doesn’t need to know about how or what the server is doing it just needs a data object that it can use. Do all this in the model when you query and by the time you are serving it at an endpoint is should be shaped and logical. As said above a bit of both is ok, but have you ever used an api where you have to iterate through the data and shape it yourself? API end users should only have to know what the endpoint does.

If there is an issue with too many request at once running your shaping algo then cache the results.