I have not seen any specialized tool to migrate from Datomic to MongoDB, no.
As always with MongoDB and similar tools, you have to know quite well exactly which queries and updates you want to perform, and how you want the exported data to be structured. If you know this, then it would be possible to create a batch job that exported the data from Datomic (make sure you use one single same db-reference/version in all the queries to get all data in sync).
I guess the best way would be to export the various documents with a query that has a pull-expression describing all the data "inside" this Document, convert this to MongoDB update data and the convert the data to JSON (this usually has to be made somewhat custom), and then import it into the Document database.
The data models and programming models are very different in Datomic and MongoDB, but I guess you are well aware of this.