It's not necessarily bad to smoke a brown cart, but the quality of the oil may have decreased due to the degradation of cannabinoids, terpenes, and other compounds over time. The brown color could also indicate that the cart has been used for a while and is losing its potency. If the aroma or taste is also not pleasant, it might be worth considering purchasing a new cart to ensure a better high and flavor. Ultimately, the risk of health hazard from ingesting burnt vapor is low, but it's still best to avoid using burnt carts if possible to limit the risk of heavy metal exposure.