What is opinion of Nichiren Buddhism on the concept of a Creator or Supreme Being? What is the opinion of Nichiren Buddhism on the concept of a Creator or Supreme Being?
Nichiren did not address the concept of a Creator or Supreme Being. Buddhists in general do not address this, basically because Shakyamuni, the founder of buddhism in India, did not address this even when asked what he thought about it.

Also, in Japan in the thirteenth century, the general population was not interested in this so no one is recorded as having asked Nichiren about it. The lack of interest may be attributed to the influence of Shintoism and Confucianism in Japan since neither pay much attention to this question either. 

In order to verify that Nichiren did not address this, the Nichiren Buddhism Library offers a searchable database for writings by Nichiren and for a record of his oral teachings as well.  

A lack of concern about a creator deity may be difficult for a theist to understand. I do know some buddhists, including some Nichiren buddhists, raised theistic and polytheistic, who do believe in a creator god. In buddhism this is okay, just like it is okay to be an atheist or an agnostic. Since these questions are not considered to be decidable, the choice of belief about a creator is up to the individual. Practicing and living one's life as a human being in this life are primary.