The narrator's initial reaction is one of disbelief and even humor, but this is because of the absurdity of the situation: a man who is a respected member of the community, and a man of science, believing in a mystical "beast" with an uncanny ability to mark people with its presence. However, as Strickland presents his evidence and the situation becomes more serious, the narrator is quick to accept the reality of the situation. He even participates in the investigation, assisting Strickland in finding the "beast."
The narrator's initial skepticism is meant to heighten the suspense and contrast with the growing evidence that leads to the revelation of the truth. The narrator's eventual acceptance of Strickland's theory is a testament to the power of evidence and the strange turn of events that unfold.
Therefore, the narrator does not pretend not to believe Strickland. Instead, he embraces the reality of the situation as it unfolds, making him a crucial participant in the investigation.