Worst coach to ever win an NBA championship?
Interesting question. Probably yes. There's 0 on his resume' outside of the Cavs, and he was basically hand-picked by LeBron.
NBA coaching is an interesting conundrum. The game is all about the players, way beyond what you see in other sports. However, if you look at the last 40 years, it's amazing how few coaches have won titles. You'd think there would be more "along for the ride" types like Lue, but there really aren't many. Guys like Billy Cunningham, KC Jones, and, to a lesser extent, Eric Spoelstra, seemed to mainly be ego-managers, but they did a great job with that over extended periods of time.
It really seems like Red Auerbach set the mold, and it's never really changed. Find the right combination of (great) players (and Auerbach gets bonus points because he did that himself), get them to buy-in as a complete team, and relentlessly beat opponents into the ground.
That was Auerbach, but also Pat Riley, Phil Jackson, Gregg Popovich, Chuck Daly, and now Steve Kerr.