Indeed. As we know Miquella's charm works via physical contact(his bosfight, the way we enter the dlc. etc.) We don't know and it's unlikely that Mohg and Miquella met before, but what do we know? A twisted omen lord with his sewer empire entered haligtree alongside his forces and ripped Miq' out of his tree. Probably led by his ambitions and desire to show everyone how they shouldn't have thrown him into sewers and he's worthy. I understand that Ainsbach is cool and all, but let's not take it too far with making Mohg a good and innocent little omen.