He starts trying to mop it up, using his human hand to shove the icing aside because gunk like that got into the joints of the cybernetic one like nobody's business and was a pain in the ass to get out. Worst case scenario he'd have to go ask Tony Stark for help if he did that.

"At least it amused you." he says with a shake of his head as he tries to salvage his cupcake, exasperated already. Bucky hates these events. Tell him to kill someone or inflitrate a high security facility, he could do that in his sleep, decorating a cupcake not so much.