We get the feeling that Rodrigo can't decide if he wants to be the narrator or the protagonist, and he definitely can't decide how he wants to tell this story. Although it's technically told in first-person (using "I"), sometimes that first-person narrator seems to disappears into an invisible, third-person narrator telling us everything a character.
When he uses first person, he's talking directly to us, and usually about deep existential ideas. When he talks about Macabéa, he seems to know absolutely everything about her: her childhood, her dreams, and even how she looks when she sleeps--even though he's apparently never met her before. That sounds a lot like a third-person narrator to us.
He also gives us a lot of information about himself. He's an affluent, sophisticated man who feels it is his duty to tell the story of this poor girl from Northeast Brazil, and apparently he assumes that his readers are similar. At least, he takes for granted that we're sophisticated enough to power through his philosophical spiels and his contemplations about writing.
Rodrigo claims he wants to tell the story as simply as possible, but what he has to say when he speaks to us is anything but simple. In fact, he can't even get himself started: "I suspect that this lengthy preamble is intended to conceal the poverty of my story, for I am apprehensive" (1.23).
This kind of self-doubt and self-reflection is typical of his narrative voice. He's ambivalent about his story, unsure about his ability to write it, and conflicted about his right to tell it. Not really what you expect from a narrator. And he also doesn't seem to have complete control over his work. When the car hits Macabéa, he struggles with what comes next, even declaring that "the hour has not come for the film-star Macabéa to die" (5.438).
Of course, he's wrong. She dies, and he goes off with his philosophizing and his writing. This is narrator who simultaneously seems to have absolute control over his work—peering into her bedroom, diverting the narrative with ruminations—and also so little control that he can't even decide what happens at the end. Sure, it's maybe a little frustrating. But maybe it also reflects the way each of us feels about our own lives.