I have set up a ZBT-2 and followed all of the Matter/Thread process described in the docs. I then used my Android phone and followed to docs to commission 3 different Ikea devices Bilresa, Kajplats, Myggbett, and an Onvis S4. They all look good and can be controlled by Home Assistant.
But when i select one of these devices in HA, select the 3 dots and choose View Thread network, I just see the Preferred Network screen and no network diagram. Why is this?
I have no other Thread controllers or networks around, but it seems as if Home Assistant is not the network controller.
It is, but you need to enable the rest nd web ports in the configutation of the OTBR addon (disabled by default) and visit the web page to see it. Don’t ask me why. Welcome to the dark side. 
Thread is analogous to WiFi, so it doesn’t have a “controller” the same way Zigbee does. Just like HA isn’t your WiFi controller, and doesn’t show you a map of your WiFi network, it doesn’t have a map of your Thread network.
If you are using HAOS with OTBR add-on as a Thread Border Router, you can view a topology map of your Thread network in the latter’s web UI. I’ll add the caveat that it’s not always correct or useful, but it’s there.
This lack of good info about the thread mesh quality bothers others as well:
https://community.home-assistant.io/t/matter-over-thread-linkquality-sensor/958067
https://community.home-assistant.io/t/displaying-network-qualitiy-of-a-thread-mesh/967745
Lets hope we get nicer info in the future 
Thanks for the info. I now have my little network diagram visible - just not where i thought it would be.
It’s clear that I need to do some more reading about this Matter (sorry )
The thread network diagram is pretty useless at the moment.
It will often not show all the nodes, not even after several reloads, and the nodes listed are pretty much impossible to map to a device that is know to the user in other places.