At first i thought it may be moulting, but it is only three months old and its little sister's feathers look nothing like it, her feathers look perfect. I would guess for about two weeks now it looks scruffier by the day?!
Whatever else is going on, that pigeon is also molting. Those are pinfeathers around their face. The plumage could stand to be preened by someone, but I don't get the impression that there's a health issue beyond that going on here.
Not an expert but it's interesting that the sibling looks fine -- maybe this one's got a little deficiency? I've seen caged budgies look that way (sad I know).