Realism can be a hard term to pin down. In a conventional sense, Cather can be considered a "realist" in that her fiction tries to convey life as it is lived, as honestly and accurately as possible. In My Antonia, the main "plot" is simply to tell of Jim's experiences growing up in Nebraska, good and bad. The major plot points tend to be small moments, such as the killing of the snake, or events that are tragic and somehow unsettling -- Antonia's pregnancy, or Mr Shimerda's suicide. Although people like to think of Antonia as a book that celebrates life on the prarie, Cather does not sugar coat the adversity of that life, and Antonia is not a romantic figure.
Another way Cather can be seen as a realist is in her handling of detail. One of her greatest attributes as a writer is her keen sense of visual detail, and it is true that certain details are especialy vivid -- the "thread of green liquid" that "oozed from [the snake's] crushed head," for example, or this passage, from Chapter two:
The earth was warm under me, and warm as I crumbled it through my fingers. Queer little red bugs came out and moved in slow squadrons around me. Their backs were polished vermilion, with black spots. I kept as still as I could. Nothing happened. I did not expect anything to happen.
Even though "nothing" happened, it is perhaps characteristic of her "realism" that such moments carry great significance. The passage concludes:
I was something that lay under the sun and felt it, like the pumpkins, and I did not want to be anything more. I was entirely happy. Perhaps we feel like that when we die and become a part of something entire, whether it is sun and air, or goodness and knowledge. At any rate, that is happiness; to be dissolved into something complete and great. When it comes to one, it comes as naturally as sleep.
Stout, Janis P.. “Seeing and Believing: Willa Cather's Realism”. American Literary Realism 33.2 (2001): 168–180.