Unlike most Maven plug-ins, Clover is a proprietary tool that stores its data in a proprietary format.
In order for the Dashboard to be able to extract Clover's results, you'll need to update the Clover report in the reporting section of your POM as follows:
<reporting>
<plugins>
...
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-clover-plugin</artifactId>
<configuration>
<generateXml>true</generateXml>
</configuration>
</plugin>
...
</plugins>
</reporting>