This is like telling someone not to be upset when their favorite lamp is broken, or not to feel disappointed when they lose a competition, or not to sweat when it's hot outside. Why shouldn't they have the most natural reaction in the world? If people like something the artist sketched in 10 minutes better than something they spent months of painstaking work on, especially if the latter is genuinely a richer, deeper, more interesting work of art, and especially if the consumers in question prefer the throwaway sketch only because it's derived from some other popular thing, like a Star Wars character or something, of course the artist might feel resentful. I wouldn't dream of denying them their right to that emotion. Coping with human superficiality is one of the necessary evils of selling art.