Read about JMS and what message types it supports. As your java object is not serializable it cannot use that, and then fallback to use text messages, which your object is converted to before sending.
You can tell Camel which JMS type you want with jmsMessageType option. http://camel.apache.org/jms
And there is a type converter in camel-jaxb that can convert java objects that has been JAXB annotated to XML, and thus what happens in your use-case.