I'm not surprised at all because Dortmund have already confirmed a place in the Champions League for next season, and it is also predicted to be an advantageous match for Mainz who have currently managed to get out of the relegation playoff zone, but the number of goals scored by Mainz is the big surprising because it was three goals without reply, even though Dortmund controlled possession of the ball, there wasn't much they could do.
Mainz benefited from the conditions faced by Dortmund, and in the last match against Wolfsburg they only needed one point to remain in their current position and make Union Berlin compete in the relegation playoff and if Union Berlin manage to win then they will remain in this division, looking at what happened to Union Berlin is quite pathetic.
On the other hand, Dortmund were fortunate enough to had played, won and qualified against PSG to the final of the UCL and although their level of play is top notch in the Champions League, it is not compared to this Bundesliga season because of the disparaging level of competition. With Dortmund making it to the finals, they have to win against Real Madrid in order to get the trophy.
For me I think their qualification to the final which has earned them a chance to get a free ticket to next season's European contest at thesame time as their position on the league table has placed them comfortably in that spot of featuring in the UCL, that's why they felt they had and have the leisure to give off outright good performance to waste. Well me thinking they'd try to stay in top form like Leverkusen just to at least gain a bit of confidence boost ahead of their clash with Madrid but they're just in their own self. Hopefully the game results well for them.