Thank you for the feedback!

Unfortunately there is a limitation in Looker Studio Community Connector API. getData method does not specify aggregation expectations for the data source. Therefore, a connector is expected to retrieve non-aggregated resultset.
I'd like to emphasize that it's not Databricks connector limitation. The same behavior can be observed with other community connectors, e.g. Snowflake connector.

We at Databricks are happy to improve the Databricks connector once Community Connector API provides relevant capabilities.