There are two types of South Asians in Japan. (Just in case you don't know, South Asian countries mean India, Nepal, Pakistan, and so on.)
1. Those who run Indian restaurants or work there.
I wouldn't say 2 are poor.
Many of foreigners living in Japan are from Korea or China, not South Asian.
The percentage of foreign "residents" (not necessarily "immigrants". Includes temporary residents like international students and expatriates) in Japan is under 2%. I don't think it can be called an "immigrant country".