The answer to this question is based on an individual reader's opinion. It is possible for a reader to believe and defend that the narrator's lack of trustworthiness actually decreases the suspense in the story; however, I believe that most readers would support the idea that the suspense is increased through our inability to trust the narrator.

The narrator alerts readers in the very first paragraph that he is somebody that readers should not openly trust. The narrator questions why we should think that he is crazy. He then tells us that his mental disease gave him extra sharp senses that allowed him to hear things that mere normal people can't hear. This is somewhat believable, but then the narrator tells readers what sorts of things he was hearing. He was hearing things from heaven and hell. That should immediately make all readers think the narrator is crazy and should not be trusted. This lack of trust in him and knowledge that he is crazy enhances the suspense and tension of the story because while we know that the narrator should not be trusted, that does not change the fact that he might not be lying to readers. Just because a person isn't trustworthy doesn't mean they aren't telling the truth from time to time. Additionally, because the narrator is crazy and untrustworthy means that readers legitimately suspect that he is quite capable of just about any horrific bit of work.