Have you tried setting quarkus.smallrye-graphql.log-payload=query-and-variables
as documented here: https://quarkus.io/guides/smallrye-graphql#quarkus-smallrye-graphql_quarkus-smallrye-graphql-log-payload
Also, in a later version of Quarkus, there should be a GraphQL log tab in Dev UI (if this is dev mode only where you need this)