are you saying Johnson voters helped Trump?
Why not? It is well known there was a high concentration of Johnson signs in the Milwaukee suburbs. Very few Trump signs. Johnson endorsed Trump, unlike Ayotte or Heck, and he was a stronger candidate than either of them. Reverse coattails are hardly implausible in this situation.
I suspect reverse coattails were much more common about half a century or so ago, when regional candidates could often strongly affect the perception of national ones (e.g., Strom Thurmond endorsed Eisenhower, leading to Eisenhower winning a great deal more segregationist areas in South Carolina than in any other state in 1952).
Some states (e.g., Indiana, Missouri) show pretty clear effects of normal top-down Trump to GOP coattails on their Senate races. Not Wisconsin.