You are correct. Currently, Servicemix plugin doesn't monitor JVM metrics. It mainly concentrate on Servicemix specific stuff mainly JBI related metrics.
I will raise a ticket to improve the plugin to include heap metrics. Currently, if you want to do it manually you can add new server from tools menu as Sun JVM 1.5 service and give it a Servicemix home as install path and once it's setup add jmx configuration. It should show up as new service and will give you JVM statistics on Servicemix Process.