The second part of the miller’s description begins displaying that “he had a sword and a buckler at his side”, which underlines the fact that he loves fighting. Right after it, the narrator shifts the attention back again to his “monstrous” physical description, comparing his “mighty mouth”, to a furnace door.
All his aspects underline the fact that he’s an horrible person and that the author doesn’t appreciate him. In fact he says that the miller is “a wrangler and a buffoon”, he generally goes to low quality taverns and he knows stories which suit very well to those places. This is understood by the fact that “he had a store of tavern stories, filthy in the main.”
Besides being a wrangler and a buffoon, he is also a thief. In fact the narrator says that “he felt the grain with his thumb and thus he knew its quality and took three times his due”. Also, the narrator says that he is “a master-hand at stealing grain”, so, probably, he cheats his customers by get their grain’s quality wrong and taking “three times his due”. After saying this the author acclaims his thumb, but he’s probably being sarcastic because of what I’ve just said and because he’s parodying the character right from the start of the description.
The author finishes the description telling his clothing (“a hood of blue and a white coat”) and saying that he carries a bagpipe that he played to carry the pilgrims out of town. So, despite the fact that he’s an horrible person, he’s creative, which is underlined by the fact that he knows a lot of stories and plays the bagpipe; but his character make this qualities almost disappear.