Little Hiroshima is a slur used to describe Asians in a derogatory way. The United States dropped an atomic bomb on Hiroshima during WW2. The association with the city and asian people in general has stuck. Refers mainly to Japanese.
The word Little Hiroshima is a racial slur that has been used historically to degrade and dehumanize asians. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to asians. If you are unsure about whether or not it is appropriate to use the word Little Hiroshima, it is best to err on the side of caution and avoid using it in any situation.