W.r.t. formatting the payload content of MQTT messages one usually sticks to standards. In case the AWS IoT platform is used the format of the payload content is not pre-defined. There's just a best practice design guide. However usually one sticks to standards w.r.t. payload content format like SensorML which is used to add meta data for distributed sensors of solar power plants for example. Are there JSON format schemas/standards for the process automation domain as well?
The Sparkplug working group from Eclipse foundation is trying to create a standard for MQTT topics and payload formating. The specification is available here. You can also find more information on the FAQ.