Dispassionate and Factual
Let's face it: living in the Republic of Gilead sucks.
Of course, it's worse for some than others, but even the most powerful don't have that great a time of it. So why sugarcoat something so awful? Even the most beautiful, flowery language wouldn't be able to disguise the way things are—it would just be another form of deceit. There's already enough of that.
And for a character like the narrator, stuck in a society that's taken so much away from her, it would be adding insult to injury to use language to mask or hide her situation, which is already so empty. Even though the narrator says at one point, "I wish this story were different [...] I wish it were about [...] sunsets, birds, rainstorms, or snow" (41.1), she can't put in things that aren't there. She can't add frippery or positive imagery.
The narrator needs to tell her story like it is, as far as she can. There's already so much she doesn't know, and so much we readers will never find out.