I think there are ways to make an artistic statement without being disrespectful. Just consider how to do it wisely, as befits someone who was smart enough to finish school.

Here's an actual graphic consideration. If part of your goal--and I'm guessing it is--is to have your artwork noticed by the audience at your graduation, you might try drawing fewer things and having one or two simple, bold graphic elements that will be more visible. A bunch of little stuff on your mortar board will simply look like clutter from the audience's standpoint and they won't be able to decipher what it is. But one or two symbols will be easier for everyone to see and make sense of. Graphic artists have to put themselves in the place of their audience and anticipate what'll work and what won't. Remember that there will be lots of folks at a distance in the coliseum or auditorium you're in, and there'll also be lots of video and still cameras. So you'll want to make the most of your message.