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.
In this situation, the main blow will fall on Kahn and Salihamidzic, because blaming Tuchel in this situation is simply pointless, he probably did everything he could (and I'm sure Negelsman would have done more and better), but that's how it is. Of course, Dortmund will win this match, victory will be won at any cost, bookmakers already give odds of 1.16 for Dortmund, everyone understands this, the importance of this victory is difficult to underestimate.