Can 'Of Mice And Men' be considered a realistic novella?

On the whole, you'd have to say yes. Of Mice and Men is meant to be, and largely succeeds in being, a realistic depiction of the plight of itinerant workers in California during the Great Depression. In writing his story, Steinbeck wanted to give a voice to those people he felt had been too long underrepresented in literature: the poor, the underprivileged, the dispossessed.
The unforgettable characters he portrays in Of Mice and Men are all the more so for being so eminently believable. George and Lennie represent many men, who in real life, were forced to travel from place to place in search of work. They have dreams, like just about everyone else in the story. But again like everyone else, their dreams somehow never seem to come to fruition. This is another aspect of the novella that makes it brutally realistic. At that time, and in that place, the American Dream still held a firm hold on the public's imagination, even among the very poorest members of society. Yet in portraying the chronic failure of any of the book's characters to achieve their dreams, Steinbeck is attempting to lay bare what he sees as the harsh reality of American life during the Great Depression: that no matter how hard you work, no matter how much you dream, the odds are stacked against those at the bottom of the heap.