Bayern Munich's performance has been erratic since the sacking of Julian Nagelsmann. But we can't blame Thomas Tuchel for that.
Tuchel did not have much time in Bayern Munich before some important matches in Champions League and Bundesliga. First he brought hope to Bayern by directly beating Dortmund. Later Bayern got unstable performances and they luckily won Bundesliga at very last minutes of the season with a genius moment from Musiala and very poor performance from Dortmund.
As you said in part of your post I cut off. Bayern Munich just wanted to bet on Tuchel to replicate his performance at Chelsea in his first season there when he won the champions league. But I am guessing that things will not work out as Bayern Munich has planned. I do not see Tuchel as a very long term coach of Bayern Munich.
To get results only is not the issue, player management and lots more could also be some issues for Tuchel. But let's wish him well and expect the best thing from them.