If anyone is going to be a candidate for the comic relief character in this play, it is going to be Giles Corey. He is the most likely out of all of the very dramatic and serious characters that we meet along the way. Arthur Miller himself, in act one, calls Corey "the most comical hero in the history" of the witch trials. So that gives us a little clue to the role that Corey is to play--that of the comic relief, of the likeable gruff old man that makes us laugh and love him because he's funny.
In act three, he makes several rather comical comments, the most memorable being "A fart on Thomas Putnam, that is what I say to that!" You know, many people in the play, and the audience, feel that way about Thomas Putnam, but Corey is the only one with the guts to express it in such a way, to the judges themselves. Then his stingy call for following correct protocol in the courts is amusing, saying that they can't arrest him for "contempt of a hearing"; they needed to be in full session to arrest him. This witty observation, and his bravery in the face of being arrested by Danforth's orders, are funny and brave all at the same time. The mood turns serious as he is hauled away however, and later, he plays a much more grave role in the proceedings. But up through act three, Corey is a rather comical figure, uttering things we think but don't say, and often getting away with it, until the end, because of his likable character and old age.
All in all, Corey lightens the mood with his gruff commentary and whimsical habits and notions; because of this, he is definitely the best candidate for a comic relief figure to the play. I hope that helped; good luck!