If it helps, it seems that mindmaps are possible with Mermaid 9.2.0 and above. Joplin 2.9.17 uses Mermaid 9.1.7. However you will see from the link that Mermaid 9.2.2 is currently being added to Joplin pre-releases v2.10.2 and above, but is currently not fully working. I do not use mindmaps so I do not know whether this is compatible with how Freemind works.