Just a few things on the possibilities on the MQ REST API.
With the MQ API facility you can PUT and GET messages on an MQ queue through a REST API. This capability only supports interacting with text messages. You will get the payload as a string, not as a “neat” JSON structure.
This is explained in Using the messaging REST API – IBM Documentation.
If you want to get a “neat” JSON API and map the “text” structure to a JSON structure and get a real API, you should use z/OS Connect.
Matt Leming from IBM explains things very clearly in this presentation REST APIs and MQ (slideshare.net)
By the way, z/OS Connect option also requires the MQ REST API infrastructure to talk to MQ.