It's a neat idea but I can't imagine I'd ever actually use this for real testing. Relying on a third party server for your tests can be a problem. We also have thousands of tests that rely on mocked REST responses, setting them up with Mocky would be a ton of work.
If Mocky could be ran onsite and had a nice API for easily generating mock responses, then I think it would be more useful.
It's an open source project, created 2 days ago. All is possible, this website is just a proof of concept.
And you can fork the project to run it locally.
And internal to that system we have some components that understand the overall shape of our data model so someData can be a very small, simple object with just the data the test cares about and the internals flesh out the rest.
If Mocky could be ran onsite and had a nice API for easily generating mock responses, then I think it would be more useful.