Yes, you would need a driver to connect to the MQTT broker, subscribe to topics, parse incoming messages (from those topics), publish to a topic, etc. Rule Machine can't do this alone due to the fact that it needs to be a device/driver, not an app, though an app of some sort would usually work in companion with such a device. It would likely be possible for them to add this if they ever add a "standard" MQTT device type, but as of now, they're just providing methods for you to be able to do this on your own.
One user is working on an app (and driver) here whose goal is to be a full MQTT client for Hubitat: [Alpha] MQTT application (currently in a private alpha, but a public beta is likely coming soon). The idea behind this would be to publish events from Hubitat devices to MQTT or to manipulate virtual/MQTT devices in Hubitat in response to incoming MQTT messages--not just read the stream and react in an arbitrary fashion. However, someone could certainly write an app that does something similar without the need for the devices (the devices created from the MQTT subscriptions; you'll always need some "MQTT broker client" device per the above). Creating devices would let you work in a more Hubitat-like fashion and would let you use things from MQTT in other apps, so it's probably a better way to go unless you have a specific reason. If you share more about your use, perhaps others will have some ideas of how to make it work.