To a degree it is not a bad idea. Landfills account for more carbon emissions that air travel, and even in the case where we collect kitchen waste it usually end up as fuel which release some amount of carbon into the air.

In addition we tend to use lawn mowers to cut grass, and chickens are quite effective at keep grass down. Replacing mechanical means that (often) run on fossil fuels with animals would be a boon to the environment and air quality. Chickens also keeps out most pests which reduces the need for toxic chemicals.

There are downsides, but on the whole I suspect it has a net-positive for the environment.