Chickens themselves are not smelly. But the environment they live in can become very smelly very quickly.
This is for two reasons. Firstly, chickens poo all the time, and their poo is a mixture of both faeces (the dark bits) and urine (the white bits). Secondly, they scratch the ground constatly, in search of food. In a chicken run this quickly reduces the ground to mud, which turns smelly once the droppings have been trampled in.
Chickens are not smelly - but the coop is, unless you clean it out regularly
The bad smells associated with barn hens do not mean your backyard hens will by stinky too. Barn hens - and the poor birds condemned to life in a battery farm - are living in cramped conditions in which the smell of their droppings soon becomes overpowering. It would be the same for any animal in such conditions, and should not be taken as a sign that chickens smell bad.
The best way to avoid having a smelly chicken coop - or fowl that smell foul - is to keep it regularly cleaned, removing the droppings as often as you can. A modern chicken shed set up such as the Omlet Eglu will reduce the chances of bad smells seeping into the fabric of the coop.