Surely the blame lies with the speech writers I'd have thought rather than with the mouthpiece they entrusted to quote their words.
That said, those writers are - presumably - intelligent people, so they must have known that their plagiarism would be discovered and, by association, that the Trump campaign would thereby be discredited.
One could almost be forgiven for suspecting it to be an inside job of sabotage...
Mind you, that said as we seem to live now in a world of post-factual politics presumably no-one in the Trump camp would give a damn in any case at any statement made in their name, however dishonest, plagiarised, racist, whatever. So long as there are enough angry, white and generally poorly educated people to drink the Kool Aid their man will sweep to power in any case