Aye, you're right. We could argue that it's not all "asians" just Hmong while the Chinese, Vietnamese, Indians etc. are hard working but that's just using tags. Let's jsut call 'em Americans.
There is a large variety of cultures in Asia, and often cultures are shaped by environmental conditions. For instance, the Chinese, Japanese, and Koreans of North Asia are considered "workaholics" by their Southeast Asian neighbors in Thailand, the Philippines, and Malaysia, and Indonesia, whom they consider very "laid back."
I think that this is as much a factor of climate as anything else. In northern regions (not only in Asia, but also in Europe and the Americas), which have four seasons, people traditionally had to work hard, economize, and prepare for the harsh winter months; whereas, in the tropical south, life was much easier-going, and people tended to worry less about the future.
Nowadays, climate is not so much a factor, but because the U.S. was settled primarily by Northern Europeans who were accustomed to harsh winter conditions, the "work" ethic has prevailed. (Contrast that with the more "laid-back" lifestyle of the Southern Europeans who settled in Latin America.)
Immigrants who can relate to the cultural values of their host country will probably have an easier time assimilating.