I also asked for a letter from an art professor, so I do think it's a very reasonable thing to do. However, I think in your case it really depends on who the other recommendations are from. If you already have several science recommendations, then I think it WOULD be nice to hear something different, from the ceramics department, Spanish department, or any other professor that will say good things about you academically/personally/etc
The only reason I would say you SHOULD NOT have the ceramics instructor write you a recommendation is if you don't have any letter from a science professor. Dental schools place most emphasis on science courses, so if you don't have any recommendation from a science professor, it may seem that you are not sufficiently "serious" about the sciences. The only other reason might be if you are much closer to the ceramics professor and are certain that they will write a glowing recommendation, and that conversely, the science professor will write some form recommendation "so-and-so was in my class and received a B, demonstrating that he/she had a strong grasp of the covered material, etc etc"
good luck!