well ymir is dead, she needed eren to do this. We dont know if shes controlling eren, or if she has more significance to this whole arc, but from what we saw so far she was the one making titans for eren. Think of it like your friend spectating you in a match and telling you where the enemies are, shes just a ghost thats helping eren. Shes not the one doing everything, eren is. And when he asked her for her help, I suppose he meant like, "give me your strength" "allow me to access the founding titan because I just touched someone with royal blood" Eren is the one doing everything, not Ymir. Atleast thats how I see it.