Maybe it makes sense to use MongoDB when you're just getting started, before you fully understand the data, so it's easy to add new attributes/columns etc.

But why even bother with MongoDB? You could just use a text editor to make bunch of JSON text files, with one record per file.

Then, once you better-understand the data, and you haven't made many "schema" changes for a while, you can set up a SQL database and import everything.