The true test of good management skills is when your people are being difficult. It's a lot less challenging and demanding to supervise a group of good, motivated people. It's only when the chips are down, when there are conflicts, resource limits, and similar strain that the real challenges of management come to the fore.
Blah, blah, blah. Everyone who has ever written anything about management has said something like this. And despite the fact that it's been said a million times, there are always people who need it to be said again. And it's all true.
However, while managing a good team might not be the gut-wrenching, talent-straining thing that so few people can do well, like managing a bad team is, it is nevertheless a skill. Yes, most people can be at least okay at it, and there are more people who are good at it, but there is still variation, and some people are excellent at it. What's more, in my experience, someone who's good at the one kind of management is not always good at the other. There are people who are great at dealing with the strain of a team that argues and resists, but who are really not that great at getting the most out of a team of motivated, happy people.
A good manager for a good team may not be great at finding ways to resolve conflicts, or to force people to do what they need to do without them feeling forced. But someone who is good at those things might not be good at finding the best organization of skills and talents to jobs, of fitting people's efforts into one another's like puzzle pieces to minimize gaps and maximize efficiencies, or at understanding the "big picture" without losing the details, enough to be the one that brings all the details together harmoniously.
No one gets much credit for being a good manager of a good team. They maybe shouldn't get as much credit as being a good manager of a bad team, but they should get some. More to the point, as long as we act as if "good manager" is a single thing in both cases, we might be missing the ball, missing a chance to improve how we do things. If you can't find that rarest of jewels, a manager who is great at both, you might be better to find ways to dovetail the talents of two people than to just settle for one. For some teams, you might be better off with a good-team-manager than a bad-team-manager, in fact. And in any case, understanding how to improve management skills needs to be based on understanding what they really are, which means understanding that each of these facets is important in its time.