This "Mundane World" is an expression we often hear, an other name for the "World of life and death" or Samsara. A world we should try to avoid because of its shallowness...
It is interesting to note that originally "mundane" means "of the world". So that really, the mundane world is the world of the world, or the worldly world...
Sitting in Zazen to experience nirvana, possibly reach great levels of spiritual accomplishments and why not save the world may seem like a worthy enterprise.
But in the end, we are rooted in this mundane world, there is no other world and this is where we are meant to daily operate.
"To return to the root is to find the meaning" (1). The mundane world, the worldly world, this is where the root is, and if we look for it in a different, non mundane world, we act just like a man trying to find his eyeglasses when they sit on his nose.