Oh, sorry if the Overlord seems to be a bit more subservient than he should, but he is quite in a weakened state. He has no place to regenerate his magic, no minions, and no adviser to help him traverse through a strange land. When he starts to regain his power, he will most likely become more bold against DIO, but the power that DIO exerts will still hang over him through the entire way of his rise to dominance.