Dortmund indeed favorites in this match, but that does not mean that this team will win easily to beat Mainz. If we look back at the first meeting of the season between the two teams, Mainz was able to beat Dortmund. That means it is something that can happen again. Because Dortmund will certainly also prioritize UCL and keep their main players from performing optimally to avoid injury. On the one hand, Mainz already knows Dortmund weaknesses. If Dortmund still uses the same tactics, I believe a draw can happen and bad result is Mainz wins again.

I would bet on double chance Mainz win or draw.