Grade-D Jackassery is Jackassery at any-level of Jackassery, while really referring to the "D", or, "Dick", and is a reference to the sexual theme of a form of Jackassery.
Grade-D Jackassery is, in essence, responding in giving your
opinion of something or someone by choosing to say option "d" - all of the above, and while simultaneously saying something was
kinky, or otherwise sexually themed
humor.