Crazy Like Us has one clear thesis: people everywhere are suffering, but the way mental illness manifests—including very real symptoms and pathologies—is moulded and guided by culture. This doesn't mean people are faking their illness, but that you cannot separate mental illness from cultural context. The second thesis is that American culture is impacting mental illness worldwide, in a strange Westernization process.
Watters makes these claims through several case studies, looking at mental illnesses that seemed to be American exports, arriving to new lands and quickly spreading. When suffering was occurring (such as from tragedies), people were not behaving according to Western clinical definitions of the mental diseases. The book is not overly critical of Western treatment, however, and remains focused on highlighting the realness and cultural context of these pathologies: "[mental] illnesses such as PTSD can be both culturally shaped and utterly real to the sufferer".
It's quite interesting, but honestly, Scott Alexander's review is just as good, and includes critical analysis as well. Start there, and pick up the book if you want more.