Stuttering is used in literature to express extreme emotion. When someone is crying, angry etc. Everyone has these types of disfluencies n their speech. This example is definitely not a punchline as the picture clearly shows a sad dragon and the disfluency is reinforcing his emotion. This is vastly different from someone marking a joke about a stuttering character. I realize that as people who stutter we may be sensitive to this but we always have to look at CONTEXT and INTENT.