I don't necessarily think that Mr. Hooper's smile symbolises anything, but I do think it reveals a lot about his character and the way he responds to the reactions of his parishioners, his loved ones and other people. Consider how the smile is described:
Mr. Hooper's smile glimmered faintly.
We know that Mr. Hooper is a gentle, kind man, for "He strove to win his people heavenward, by mild persuasive influences, rather than to drive them thither, by the thunders of the Word." We can thus see his smile as his ironic and gentle response to the lack of understanding that those around him have of his reasons for putting on the black veil. For Mr. Hooper has learnt the terrifying truth that all characters at least acknowledge in part if not completely - that all of us bear the black veil on us with everyone, and the veil will only be taken away when we meet our Maker.
I agree with the previous answer. I do not believe that Mr. Hooper's smile is symbolic of anything. The veil is the predominant symbolic image throughout this story.
Mr. Hooper's smile is not an emotionally happy smile, though. In fact, it is frequently described as "sad." The smile is also described as "faint" several times. Hooper is not grinning ear to ear with pleasure. His smile is a smile of irony. He knows the reason that he is wearing the black veil. It is symbolic of secret sin. He also knows that every other person also wears his or her own metaphorical black veil. Hooper smiles because he knows that there is a sad irony in the fact that the people in his town do not understand the meaning of the veil.
Hooper constantly has a faint, sad smile that glimmers beneath the veil because he's witnessing similar naivete. People don't publicly acknowledge their secret sin, but it will be exposed in the afterlife.
A sad smile gleamed faintly from beneath the black veil, and flickered about his mouth, glimmering as he disappeared.