Comic relief is "a humorous or farcical interlude in a serious literary work or drama, especially a tragedy, intended to relieve the dramatic tension or heighten the emotional impact by means of contrast."
Comic relief serve the basic purpose of allowing the audience time to relax and escape from the tension of particularly suspenseful or intense scenes. Most comic relief, therefore, is obvious. Because it is designed to contrast highly dramatci scenes, it stands to reason that the humor must be explicit, so has to balance the dramatic scene. In addition, many members of the audience at the Globe and during Shakespeare's time, was not intelligent enough to understand implied humor.
However, come would argue that some of the humor in Shakespeare is implied. Particularly evident are the jokes of the Porter concerning male "performance" after drinking. Yet, again, the audience at this time may have understood this more clearly. It is hard to tell which elements of comedy were more obvious to audiences 400 years ago.