Cervantes's Don Quixote is one of the earliest examples of a work of metafiction. Metafiction is a self-referential literary style in which the author consciously draws the reader's attention to not just the story itself, but also the way the story is told. Cervantes does this in Don Quixote by positioning himself not as the author of an original work of fiction, but as the translator of a work of historical nonfiction originally written by the (fictional) historian Benengeli. The conceit is that Don Quixote is a real historical figure, Benengeli is the historian who recorded his life story, and Cervantes is the translator taking liberties with his own version of events.

This device, which in places functions almost like a frame tale, allows Cervantes a degree of narrative distance from the events of the story. He is able to insert his opinions and snide commentary about events into the story from the position of an objective third party because, within the universe of the novel, that is exactly what he is. For example, Chapter 5 begins,

The translator of this history, when he comes to write this fifth chapter, says that he considers it apocryphal, because in it Sancho Panza speaks in a style unlike that which might have been expected from his limited intelligence, and says things so subtle that he does not think it possible he could have conceived them; however, desirous of doing what his task imposed upon him, he was unwilling to leave it untranslated, and therefore he went on to say.

This preface is quite clever because, in dismissing the chapter for being unrealistic, Cervantes the translator (and character in the novel) allows Cervantes the author to get away with writing an unrealistic chapter. Cervantes also uses this technique to skim over details, as in Chapter 17 when he writes,

Here the author describes minutely everything belonging to Don Diego's mansion, putting before us in his picture the whole contents of a rich gentleman-farmer's house; but the translator of the history thought it best to pass over these and other details of the same sort in silence, as they are not in harmony with the main purpose of the story, the strong point of which is truth rather than dull digressions.

The effect of these asides from the translator is often quite humorous, as it is this distance between teller and subject that keeps the reader aware that, despite what the man himself believes, Don Quixote is not actually a knight-errant.

Cervantes, as the "translator of the work," addresses the reader directly on several occasions. Most notably, in the prologue to part II of the novel, Cervantes takes the time to complain about an unauthorized sequel to part 1 and to urge the reader not to believe any secondary stories of Don Quixote, as they may come from other, less reputable, sources. Cervantes goes so far as to instruct the reader on what to say should they ever encounter the author of this false sequel. One effect of this direct appeal to the reader is to create a sense of intimacy between translator and reader; Cervantes is telling this story to you specifically.

While much of this metafictional technique is used to make the text apear to be a fascimile of an actual historical document and therefore instill in the reader a sense of realism, the quotes in your question above represent two vastly different points of view on Cervantes as a translator. That there is contention within the text itself as to Cervantes's skill as a translator and as to the exact events of Don Quixote's life creates doubt in the reader as to what they should believe at face value and what they should question.

As Don Quixote is very much a book about books, with Alonso taking on the persona of Don Quixote in order to be more like the characters in his favorite histories, it makes sense that Cervantes would want to draw the reader's attention to the act of telling itself and would want to force readers to question their faith in the author/translator and the very events of the story itself.

How does Cervantes's translator affect the reader's perception of Don Quixote?

This question has already been answered in great detail on eNotes. I'll include the link below.

The short version is that the "translator" character in Don Quixote is Cervantes himself, inserting himself into the plot of the story by pretending that he is not the author but instead merely a translator of a much older text. This metafictional conceit gives Cervantes the ability to comment on the text as though he is a impartial third party. For example, Cervantes the translator opens chapter 5 by saying he does not believe the characters of the story are behaving believably in the chapter that follows, and he wishes he could have excluded it. This disclaimer adds to the humor of the text, as does the fact that the reader can never be sure how much to trust the translator. The two quotes in the question you posted are alternate descriptions of the translator: is he scrupulously honest or a lying dog? The reader can't be sure and so must take everything he says (and therefore the whole of the story itself) with a grain of salt.