Hi JohnAN - MarsEdit stores all of its data in a folder in your home folder. To open the MarsEdit data folder, hold the option key on your keyboard while selecting the Help menu at the top of the screen, then select “Open MarsEdit Data Folder.”
As long as this folder is backed up then all of the posts that are synced to your Mac, and your local drafts, should be safe.
I probably wouldn’t rely on MarsEdit as your only backup, though. It would be better to arrange for some kind of regular backup of your blog from the server itself. In the case of Micro.blog you might read up on the options to export your data: Exporting from Micro.blog
Ideally you would have a backup of MarsEdit’s data AND a regular backup of your exported data from the blog.
Daniel