Yep. It's not meant to be efficient, it's meant to be a garbage can. It's 4 items per coal. Except trophies, trophies yield 1:1 ratio.

The obliterator becomes a much better source of coal once you are farming massive amounts of crops. I wouldn't recommend barley, because barley converts down better to bread, a decent trash food. But extra Jotun Puffs, Mage Caps and Flax can all be fed to the obliterator to yield large quantities of coal, depending on the size of your farm.