Mind mapping is more of a process, rather than a data structure. The process helps us brainstorm, recall, and visualize our ideas.
You're right in that some of the data structure behind mind maps can be represented using a tree. However, you'd lose lots of the benefits. For example, trees might not be as memorable or as unique as a mind map[1].
[1]https://imindmap.com/blog/mind-mapping-for-memory-why-is-it-...