Yes, the data coming from the Piwik API belongs to you and is not covered by the GPL.

You can use Piwik as a service, and reuse, publish, display, modify, process the data coming from the Piwik API using the REST API web services call. See also the GPL FAQ: Is there some way that I can GPL the output people get from use of my program? For example, if my program is used to develop hardware designs, can I require that these designs must be free?