First, Johnson is a movie stars. How many movie stars lend their voice to animated television shows unless, you know, they were on their way to not being movie stars anymore? Second, if the Rock was lending his voice to this series, he should be a heavy-hitter character. Cliffjumper is anything but. On the other hand, the Rock will likely make Cliffjumper sound better than Casey Kasem ever did.
The only thing I can figure is that Johnson must have been a fan of the original cartoon in the 80's. He sure as heck doesn't need the pittance of a salary they will likely pay him for this.