Not as far as I know, but I guess the pieces of the puzzle are there. You will have to do a lot of work by yourself. The future Graphql-module for Mendix is very likely pretty similar to the OQL module. Take that as a start and you might be able to generate the module by yourself. The OQL-module is the closest thing to Graphql-support that Mendix has got at this moment.
application/json content type – So, you can utilize Mendix’s REST Service to consume or produce.
The capabilities that you mention in your comment, minimize service calls & control over data requested, are also provided by Odata. Mendix currently already provides that to some extend, for example if you have a data grid with associated attributes, these will be fetched in a single call by using the $expand feature. Specific attributes are requested using $select.
Goal with mendix external entities is to provide similar performance optimization like GraphQL, but completely automated: mendix determines the query based on what you have on your page (including sorting, paging, filtering), the service will automatically parse this query and translate it to SQL statements to fetch the data from the source database.