If I have data that I will only need to update very rarely (once a month), would it be a good idea to use a JSON file instead of a database table with only 1 row?
With a database approach you can relate that line with the others in the database. With the file you'll need to read it and initialize a CLOB to process it.
In other words, the data will exist for the database.
Other thing to consider is that with a database comes more things that just storage of data. You get logging, transactions, permissions...
Analyze other aspects of your application to be sure that you really don't need those.