The words "good" and "bad" can be thought of as subjective, but not here. If you say that using the words "good" or "bad" automatically makes it subjective, then there would be no such thing as a "good" or "bad" video-game, they would all just be games. There would be no "good" or "bad" movies, they would all just be movies. You couldn't have "Good" or "bad" teeth, you would just have teeth. And so on and so forth. Its all about having a mutual understanding of what you mean by "good" or "bad".
Also, saying that the only element require for a prequel is for it to proceed a story, is like saying the only requirement for a video-game is for you to be able to control digital elements on a screen. That means if I make a game with horrible frames per second, a cliche story, and gameplay that is unpopular. I'm still, by that definition, making a "good" game.
When you say something is "good" in literature, you can be asking multiple things. Here we are asking did Skyward Sword followed the correct structure that is known for making prequels. One major point, perhaps the major point of making a prequel is not just to have a story before a story, but to answer questions. A lot of times, people leave mysteries in their tales just so the prequel can answer it. SS did not deliver in answering the questions that the series had established. A prequel with many unanswered questions is by definition, a bad prequel.