outputFormat Required Specifies the desired output format: pdf, html, xls, xlsx, rtf, csv, xml, docx, odt, ods, jprint.
Currently there is no JSON exporter in JasperReports Library. But is extensible so if needed you can create your own exporter. See http://jasperreports.sourceforge.net/api/net/sf/jasperreports/engine/exp... the CSV exporter will be a good place to start. You can also add an enhancement request in the JR library project http://community.jaspersoft.com/project/jasperreports-library
But using a full report execution just to receive raw data may be an overkill, If you just need raw data in JSON you can use JasperServer Domians and the REST Query Executor service, see http://community.jaspersoft.com/documentation/jasperreports-server-web-s... there is a nice sample of this here: http://mdahlman.wordpress.com/2013/05/07/rest-endpoint-for-jaspersoft-qu...
Note: Domains are available in the Pro version of JasperReports Server