Not really true. Yes Mainz stayed solid in the Bundesliga but back then Tuchel definitely had a harder task at his hands than when he left (less financial support, had to get them promoted first).
As
@hasanejaz88 has pointed out, Dortmund have definitely been worse. And I also argue that PSG have gone backward,
they failed to win the league after he left and as of now he's the only one to ever get them into the CL final.
The guy is clearly an incredible good manager but obviously a massive pain in the arse to work with, the whole clashing with Boehly doesn't surprise me at all.