She clearly violated 18 U.S. Code § 871 by threatening the life of the president. Being a celebrity or claiming you were "just kidding" is not an excuse.
Did Kathy Griffin break the law with her bloody Trump head video? That comes down to a question of whether you believe there was a real threat - expressed or implied - in the video.
Threats don't require words, after all. Threats can be expressed with images. They can also be implied with images.
If you haven't already seen it, here is the video that has unfunny comedienne Griffin in so much hot water: