But I see them as favorites in this match.
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.
If you look at the performance of both teams in the last few matches, it is clear that Mainz is slightly better, only Dortmund will play at home in the match later so they are slightly favored, but I am also not sure that Dortmund will be able to win the match because Mainz is in great motivation to be able to finish in the top 4 of the Bundesliga standings this season, especially now they are in 4th position in the standings with quite fierce competition.
For betting options, I think it is quite safe to bet on Mainz's double choice or a draw or even betting on over 2.5 goals is not a bad choice in this match.