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.
Three Wrong Views from the Flower Garland [Kegon or Avatamsaka] Sutra
"In this world there are three wrong viewpoints. If one clings to these viewpoints, then all things in this world are but to be denied. First, some say that all human experience is based on destiny; second, some hold that everything is created by God and controlled by His will; third, some say that everything happens by chance without having any cause or condition.
If all has been decided by destiny, both good deeds and evil deeds are predetermined, weal and woe are predestined; nothing would exist that has not been foreordained. Then all human plans and efforts for improvement and progress would be in vain and humanity would be without hope.
The same is true of the other viewpoints, for, if everything in the last resort is in the hands of an unknowable God, or of blind chance, what hope has humanity except in submission? It is no wonder that people holding these conceptions lose hope and neglect efforts to act wisely and to avoid evil.
In fact, these three conceptions or viewpoints are all wrong: everything is a succession of appearance whose source is the accumulation of causes and conditions."
from the Avatamsaka-sutra....