In his contribution to Essays on Skepticism, Relativism, and Ethics in the Zhuangzi ("Was Zhuangzi a Relativist?"), Philip Ivanhoe observes that one cannot understand Zhuangzi's ethical stance without first understanding his view of human nature. Citing Graham, he suggests that this was a major topic of debate of his time and thus one to which Zhuangzi must certainly have given considerable thought. Given this, it is interesting that no one other than Ivanhoe and Graham (that I've read) addresses the issue from Zhuangzi's perspective. But then again, this should not surprise us given that Zhuangzi does not seem to address the issue head on at all.
Certainly, we can come to some understanding of his point of view regarding human nature since it is implicit in his philosophy, but he does not seem to have thought it necessary or even advisable to deal with it as a discrete issue. This, I think, is at least as important to our understanding of Zhuangzi as is whatever conclusion we in the end draw. Whatever that view is, it is something that emerges consequent to his description of human enterprises, not something that explains those enterprises. In contrast to many others of his day, Zhuangzi eschewed the use of definite 'principles' to describe reality. Rather, he let the human experience speak for itself. "Adding nothing to the essentials of life" is precisely this — letting life express itself and learning thereby how best to live it.
By contrast, Mencius, who declared human nature good, and Xunzi, who declared human nature evil, both begin with a template and apply it to life. This is exactly what Zhuangzi sought to avoid; life requires no explanation; the best way to acquire an understanding of the best way to live is to live in awareness of how life lives. This contrast illustrates the radical divide between a rationalistic approach to life and an organic, phenomenological approach. The former allows that things can be put in tidy little boxes which in turn enables a prescriptive ethics; the latter leaves things in their native messiness and ambiguity.
Ivanhoe suggests that Zhuangzi saw human nature as "benign". By this I believe he means neither good nor evil, though not necessarily neutral. It means that when a healthy individual exercises his 'natural inclinations' he will naturally not harm others. This differs from the point of view of one of the contemporary interpretations he critiques (Wong's), namely that Zhuangzi saw the natural outcome of being natural as to experience compassion for all things. I think he makes a convincing argument against this view from the perspective of the text, though not necessarily from the perspective of experience. Zhuangzi, I think, would be sufficiently pleased if we just had the experience to interpret; the nuances of the interpretation are secondary and relatively unimportant.
You can check out Scott's writings on Zhuangzi here.