It's not because Miko-chi is stronger Ui-mama, It's because Ui-mama is too weak! And it's one of the many things we love her for! ^_^ Also I wonder if Miko-chi will put Ui-mama's face in the Room? lol.
It depends on what you category you count, like number of viewers live, subscribers count or hours viewed. I think biggest active indie Vtuber is Filian if you count subscribers number. Live viewer number may favor UI tho. So the answer would be depend.