I think Yi Deng's intelligence is grossly underestimated. He was able to learn Yi Yang Zhi to a supreme level (possibly the most powerful of the Greats next to WCY at Hua Shan Luan Jian 1). He governed a country that was quite sizable and powerful in it's time. He was well versed in art and poetry (as shown in his conversation with Guo Jing and Huang Rong when they first met). He also have considerable medical knowledge (as when he healed Lu Wu Shuang). Certainly, Huang Yaoshi was more inventive than Duan Zhixing/Yi Deng. However, Yi Deng was able to accomplish just as much with far more responsibility in his hands.