Hi @johnnyngena, no, the Meraki Dashboard only contains metadata about a customer's network traffic and does not contain any actual customer data. You also have the ability to disable the traffic metadata collection from the Network Wide > Configure > General page in Dashboard, if desired. So any customer data is only limited to the operational configuration and connection information collected by the network devices. If you are referring to any potentially sensitive data such as the PSK for an SSID or shared secret for a RADIUS server, all such data is AES-256 encrypted at rest on the cloud controllers.
Your question is too open and wide of scope to answer easily.
Are you referring to direct customers of Cisco Meraki who own the actual Cisco Meraki devices, or customers of a business that use Cisco Meraki?
Obviously data that you type into the dashboard is stored there, such as network names, organisation name, device names, device locations, organisation and network admin email addresses, etc.
If you use Meraki splash page authentication for the customers of the business that owns the Cisco Meraki kit (such as a cafe) with Meraki hosted authentication then their email address (which is their login details) are stored.
Perhaps you should take a look at the Meraki Trust section of the web site to see if it can answer more specific questions.