Yes using API to be configured as a data source for lookup is very useful feature that should be in Forms.

Now most of the customers and systems doesn't provide direct access to the database through tables or views, instead they are providing only web services for security.

I hope I can see this feature in Forms soon as we have a lot of customers requesting it.

No, Forms lookup rules and data source only support data from database (Microsoft SQL Server or Oracle).

Hey, I know this is an old question, while it can't be loaded in as a data source, a query can still be ran on it with JQuery using JQuery .getJSON

See code example below for how we implemented this on our own project.

Also note that you may need to pop the code out into the separate pop out option to read it better due to the length of the API URL

  //q61 is the input box for the zip code used for Querying API
  $("#q61 input").change(function(){
    
    var currentYear = (new Date()).getFullYear();
    var currentZip = $(this).val();
		$.getJSON( "https://inventory.data.gov/api/action/datastore_search?resource_id=8ea44bc4-22ba-4386-b84c-1494ab28964b&filters=%7B%22FiscalYear%22:%22"+currentYear+"%22,%22Zip%22:%22"+currentZip+"%22%7D", function( data ) {
			//Code your trying to accomplish with API Query goes here
          });
});