Integrating the whimsical nature of the sugar skull with other tribal and artistic influences has given birth to a postmodernism quality in present-day sugar skull drawings, paintings, and tattoos. You don't have to be Catholic or even Christian to appreciate the artistic value of the sugar skull. On paper, the decorative shapes and lines can be even more intricate than on real sugar skulls piped with thick, sticky icing. Artists have enjoyed the use of the skull as an artistic platform for centuries.
While sugar skulls created purely for artistic (rather than religious) reasons may have developed their own identity separate from the Catholic/Mexican culture they came from, I think it's still important to recognize and respect the history of these beautiful, cultural artifacts. If you want to get a tattoo of a sugar skull, there really isn't a defined set of rules for the design. The shape of the skull itself can be the more traditional squared chin shape or the more exaggerated Oaxacan style with the high, protruding cheekbones. The skull can be decorated with pretty much anything you want: flowers, swirls, stitches, geometric shapes, stars, dots, lines, etc. If you're having trouble coming up with ideas on how to adorn your sugar skull, your tattoo artist can help you decide based on things that you like. As with any tattoo, the only rule of thumb is to never copy someone else's idea. Come up with your own and it will have a lot more meaning for you in the long run.