In the prologue of "The Canterbury Tales by Geoffrey Chaucer, the author draws almost the full gamut of human weaknesses and frailties based upon our understanding of the seven deadly sins. For some characters, such as The Wife Of Bath, he allows a little tolerance it seems - but not for the poor old miller! In the Miller's Tale, Chaucer deliberately sets out to describe the crook in almost bestial terms, to demean him. One possible reason for this could be that in Chaucerian times , millers were often mistrusted, almost to the point where being a miller became synonymous with being a cheat or swindler. Everyone was dependent on millers for basic bread and grain, so they weren't easily forgiven!!
It is pretty clear (assuming that you are asking about the General Prologue) that Chaucer does not really approve of the Miller. He thinks he is a cheat and something of a ruffian.
You can tell he is saying that Miller is a cheat because he says his thumb is golden (he puts his thumb on the scales to cheat his customers). He says he would charge three times the right price and would steal corn.
His physical description of the Miller is pretty unpleasant too. He compares him to a fox or a sow. He says how big his nostrils are. He talks about how big his mouth was. So he generally just makes him sound kind of gross.
The description of the miller from the Prologue is a good example of why Chaucer continues to be popular -- Chaucer is a master of finding the key, telling detail. He's also very funny. The miller, for instance, is a loud-mouthed bully who cheats his customers and steals grain. Chaucer begins by giving a physical description of the miller -- he is "big in brawn and bone," with a red beard like a shovel. There is a wart at the end of his nose with bushy hairs "red as the bristles on an old sow's ear" growing out of it. Chaucer describes his wide, black nostrils and his "mighty mouth" that was like "a furnace door." So the miller is personally not very attractive. The miller's character as a braggart and bully also comes out: he boasts that he can batter down any door (by ramming it with his head!); he knows plenty of dirty stories; he can judge the quality of grain with his thumb, which makes it easy for him to steal the best grain. The overall impression we get of the miller is that he is greatly impressed with himself and he loves to be the center of attention. No doubt he thinks himself to be very funny.
Chaucer sees through the miller. He's not a good person, and Chaucer is clearly lampooning him, so in that sense he "disapproves" of him. But Chaucer casts a critical eye on everyone -- part of his point in The Canterbury Tales is that everyone has a human flaw or two.