I don't think the charcoal output is too high but I do agree that the tar output could use an increase. Then you can just build less tar kilns if you don't need as much charcoal. If your village is serious about industry, 12 per smelter will start adding up quite fast and eventually you will move in to making wrought and steel as well which adds to your usage.
In addition to this, it seems tar kilns are meant for bulk where as mining will be for better quality. You're going to find it pretty difficult getting that same amount of charcoal out of your tar kiln every single day at a decent quality when you try to fill it with only high quality wood.