Borussia Dortmund only needs 1 more win in their last match to ensure themselves as Bundesliga champions this season, Mainz, who will be against Borussia Dortmund in his last match, is not so strong in terms of quality when compared to Borussia Dortmund, so it's only natural that Borussia Dortmund is highly favored.
Whichever other team to be in place of Mainz,even Bayern Munchen themselves I think the level of motivation of Borussia Dortmund knowing that a win can give them a Bundesliga title would be enough for them to win that game.The fact that they are playing at home against Mainz,a mid level team in Bundesliga means that with 99.99% assurance they most likely will win this game and become Champions of the Bundesliga.
All the blame for Bayern losing the title this year will fall on Bayern management and their changing of coach in a critical moment for the team,I am sure the Bayern fans will not be happy at all at the end of this season and they have the right to ask for explanations.