Deng Xiaoping's reforms were largely responsible for modernizing China's economy. It was Deng who moved China away from a centralized economy and toward the more open and dynamic economy that it has today. It was Deng, after all, who was quoted as telling the people that "to get rich is glorious." This represented a major move away from the policies and ideas of Mao Zedong.
One can argue that Deng's reforms have hindered modernization because they enhanced the power of the army and did nothing to promote a more open government. This was seen especially in the aftermath of the Tiananmen Square demonstrations of 1989. Because Deng did these things, China's government is still quite repressive even though its economy is modernizing.
In this way, Deng's legacy is somewhat mixed in terms of the extent to which it modernized China.