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

You can use Matomo as a service, and reuse, publish, display, modify, process the data coming from the Matomo 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?

Previous FAQ: Can I use Matomo and provide analytics Hosting service? What are the requirements for Matomo Hosted as a service?