That was my line of thinking at times too but in all honesty Dortmunds league form never once impacted their CL form, if anything the different competition let them escape the troubles of their domestic fuck ups.
The problem for Dortmund is, when our defence is not making blunders and we have no reason to throw people forward we are quite disgusting to play against, and Dortmund with all their pace and all their energy have even less creativity than us on a bad day. So when they meet us, playing a way no other German team physically can, it throws them off their game completely.