We do not support CPU and memory metrics on our managed Kubernetes Dashboard at this time. Unlike the upstream Kubernetes, DigitalOcean does not deploy the metrics-scraper on the managed Kubernetes Dashboard, which is why the CPU and memory usage metrics are not available on our dashboard. If you want to see those metrics, you can deploy an unmanaged dashboard instance, as described in the Kubernetes documentation.
Our team is working to support metrics in the Kubernetes Dashboard. You can follow this issue to get notified when we support this feature.