Well, when the Raven appears at his lattice, the narrator says the bird made him smile:
Open here I flung the shutter, when, with many a flirt and flutter,
In there stepped a stately Raven of the saintly days of yore;
Not the least obeisance made he; not a minute stopped or stayed he;
But, with mien of lord or lady, perched above my chamber door—
Perched upon a bust of Pallas just above my chamber door—
Perched, and sat, and nothing more.
Then this ebony bird beguiling my sad fancy into smiling,
By the grave and stern decorum of the countenance it wore,
“Though thy crest be shorn and shaven, thou,” I said, “art sure no craven, Ghastly grim and ancient Raven wandering from the Nightly shore—
Tell me what thy lordly name is on the Night’s Plutonian shore!”
Quoth the Raven “Nevermore.” (stanzas 7 and 8)
Given the tone of the poem (despondency over the "lost Lenore" and dread and alarm over the incessant tapping and rapping) it's natural to wonder what there is for his "sad fancy" to smile at. Clearly, the bird is not comical! One way to think about it is that the bird, by squatting on the "bust of Pallas," the goddess of wisdom, is seen by the narrator to be making, unintentionally, a kind of ironic comment on his "work," which is to forget Lenore by throwing himself into his studies.
Another way to think about it is to consider the narrator's state of mind. His "sad fancy" is his predeliction for imagining gloom and terror. Of course, in such a state, he would be visited by a raven, a bird of ill omen. Perhaps his smile comes from recognizing the inevitability of the Raven's coming, which is an omen of another inevitable event, his death.
The speaker was rather surprised and bemused. In the first two lines of the eighth stanza, he says, "Then this ebony bird beguiling my sad fancy into smiling...By the grave and stern decorum of the countenance it wore". The sight of the bird, sitting so seriously above his chamber door with an almost human-like demeanor must have been a ludicrous sight, and it made him smile.