There should be ways to easily jump to the source code for a Maven plugin used in your build. The IDE knows the exact plugin artifact coordinates, it can look up the mojo class name in the plugin’s bindings, and it can use SourceJavadocAttacher to find sources when necessary. (From there, you can easily use Ctrl-B to navigate to various dependencies.)
1. Hyperlink a <goal> in the POM editor.
2. Hyperlink the part of a line in the Output Window showing a goal being executed.
I'm not convinced the majority of users need to/should read the sources of maven plugins. Maybe debug maven build when thing go wrong (separate issue is filed for that), but why navigate the source only? s
Hard to say what other users need, I just know I have wanted this on a number of occasions. For example, some mojo parameter is poorly documented and I want to see how the value is actually being used.
Currently you need to go to the Maven node in Services, search for the plugin you are using, pick out the right version, download its sources, and browse to the mojo.
@Jesse: It is easier to invoke "mvn help:describe -Dcmd=<goals> -Ddetail" to get the mojo parameter for the plugin instead of downloading the sources. See 
@Milos: The output of "help:describe" could be parsed and provided somehow (code completion, tooltips, separate view). What do you think?
done as popup menu action in Build execution overview dialog (triggered from the output tab toolbar)