Dortmund are never a credible team, at least in this season especially when matched with teams of relatively equal strength, you find Dortmund fooling around on the pitch like they did yesterday. I'm so much scared when taking a bet for them in the bundesliga, the closest team to credibility in the bundesliga after Bayern is Leipzig. These guys are just good but not unstoppable though but they are a team you can bet your money on and remain calm about it.
What they played in this season is same to the last season of Aubameyang in Dortmund and I hope this season won't trigger another transfer wave in Dortmund. When stars see no future, they will leave.
Honestly Dortmund wasn't bad all season, they just became bad lately and that's it, they were quite good if you ask me but ended up not being that great for the past periods. In the end we should not be really shocked with teams having ups and downs because Bayern ends up dominating the league so much that teams go all out and try to make a big splash in order to beat Bayern, when they fail they have some recovery seasons and try back again.
You can't hope to beat Bayern by just playing a decent football and improving your places a bit every year and not even every position, just gradual increases to reach Bayern level, they have so much money and do so much changes that they literally end up with the best team possible in the end even if you make gradual changes, so teams make big huge changes and sometimes it helps them get closer and sometimes what happened to Dortmund happens.