Can dark green stools that don't smell bad be from an unhealthy diet in teenagers?

It's possible, but: not necessarily the case. The color of the stool depends on several factors, but most importantly, the time it takes for food/substances ingested to move through the intestinal tract. (which changed the amount of bile excreted into the intestines and then reabsorbed); the faster substances move through, the greener the stools. Some stools that are bright green are from infections.