I agree. It isn't just a Cora thing, it's the entire MLB (and perhaps organizational) coaching staff. Cora, in my opinion, takes the largest share of the blame because he is the boss, and that's how life works. Unless, however, his job description from the top is "look bored in the dugout and talk to the press so we don't have to", which, well, I am not entirely sure isn't the case.
I can't think of a single player who has come here in trade or as a free agent and gotten markedly better, meanwhile, Brasier moves to LA after a month off and is immediately turned into a high caliber arm. Jeffrey Springs in Tampa before his shoulder fell off. Among others.
On the other hand, Llovera comes over from San Francisco and stinks. Kluber (older and washed up, for sure) signs and immediately starts walking almost 4x as many people as he had in his career.