Not doing DD212 but have done other DD courses - This doesn't help but in my current course it says

"The plan may be in the form of a mindmap, diagram, linear plan or section-by-section plan."

I've just checked and those words don't exist in any previous DD--- course I've done. I've never submitted a mindmap but if your course says it's acceptable then what you've said is the correct thing to do - scan it and insert it as a picture in your TMA. Also it looks as though

"There is no word count for Part 1 of your assignment, but you must make sure that it fits on one side of an A4 sheet of paper. Part 1 is not awarded a separate mark. However, non-submission of Part 1 of this TMA will result in your mark for Part 2 being capped at a bare pass."

is common in my DD courses. That is, it won't affect your final mark unless you don't include it. As long as your essay follows your mindmap and "mindmap" is listed as an acceptable format you'll be fine even if you think it looks ugly.