While he was no saint, don't forget that in his times, much more was at stake in foreign policy than the reputation of the U.S. Just 7 years before Nixon came to power, the world almost faced a catastrophic nuclear war during the Cuban Missle Crisis. The détente under Nixon and Ford at least meant that direct confrontation between the Soviets and the West became a lesser threat.
As for his support for bloody coups and not-so-democratic regimes, please don't forget that at that time, "winning" the cold war was seen as a matter of life and death (and losing it certainly was the primary reason behind the Soviet Union's demise), so it's not surprising that he wasn't particularly picky when he chose his allies. And winning China over was certainly a diplomatic feat that he can be proud of.