The rats in NIMH represent characteristics that are expressed by human beings regularly. Ms Frisby us a widow, therefore she must find a way to survive. This is the challenge that is faced by many women whose husbands die. What makes matters worse is that her house is about to be destroyed by a plough. The fact that she is a mouse is not important. What she is going through, again, is something that happens to human beings all the time when they are faced with the loss of their homes due to foreclosure or government entities taking real estate to build roads. Making matters worse, her sick son presents another challenge often faced by single parents - work or take care of a sick child.
Mrs Frisby seeks the aid of the rats of NIMH. Ultimately, they become heroes, saving the day. This draws a parallel to the human idea that you don't have to be big and powerful to be a hero.
The themes of the book all relate to social issues that are prevalent in American society. The author has used a literary device that is common - casting animals in human roles - in order to tell a story in fable form that crafts a moral or social lesson.