Because your domain model is defined Input *-1 JsonObject. This means:
Therefor it is not possible for the mapping to determine what Input-object to map the JsonObject-object to.
Let Mendix do the work of creating the importmapping for you:
The outcome of the microflow is a JSON-string that you can feed to your importmapping. Probably you will see a difference with the JSON that you currently try to import.