Conversational and Informal
"The Moths" is written in stream of consciousness, where the reader gets to find out what the narrator is thinking as she thinks it. Sometimes stream of consciousness can make texts a little chaotic to follow—think about how much leaping around your own brain does—but in this case, the result is pretty chill.
For instance, the narrator will tack a sentence fragment onto the end of another sentence to clarify a point, just like you would in an unedited chat with a friend:
"I was only fourteen years old when Abuelita requested my help. And it seemed only fair." (1)
As readers, it gives us the feeling of hearing the story more than reading it, of peeking in on someone's unedited thoughts instead of pouring over their polished version of events.
That informality carries over into the way the narrator says things. She talks like a kid, not like a snobby professor or a beauty pageant contestant:
"Not that I was her favorite granddaughter or anything special." (2)
Fragments like this one give the whole story a very intimate feeling; if it were told in formal prose it would seem distant and edited, and since it isn't, we feel like we're getting the real deal as the story unfolds.