No, Montresor is not a trustworthy narrator. It is because he has narrated the story in justification of his act of murder, thus the readers can not commend him to be credible or trustworthy.
Explanation:
"The Cask of Amontillado" is a short story written by Edgar Allan Poe. The story is told from Montressor's point of view.
Montressor narrates the story of how he  planned and murdered Fortunato. He can not considered reliable narrator as he is manipulative, which can be seen in his manipulation of inviting Fortunator to his house and also in the way he manipulates his servants to leave the mansion. The story is told from self-sympathetic point of view of Montressor, who thinks that he is been wronged by Fortunato. His narration can not trusted as he has appeared to be a liar to Fortunato and his tale can not justify his act of burying a person alive.
The evidences of unreliablity of Montressor's narration found in the story are:
"The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge." (Poe 1)
This shows that Montressor's motive has been to revenge on Fortunato since the beginning of story, which makes him unreliable, as he is narrating the story to justify his act of murder.
"There were no attendants at home; they had absconded to make merry in honor of the time. I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned." (Poe 2)
This statement is evident that Montressor is manipulative and thus can not be trusted.
i hope this helps!
