I think you misunderstood, I wasn't talking about quality at all, I was just talking doing a bad job adapting the source material.
"Bad" is a judgment of quality. And you're making the wrong assumption about what the purpose of an adaptation is. The word "adapt" literally means to adjust or modify something to suit a new context or purpose. What makes something a good adaptation has nothing to do with how similar it is to the original. The original is not the goal you're aiming for -- it's the starting line, not the finish line. The goal you're aiming for is a worthwhile new version. You do a good job of adapting something if the adaptation itself is good.
After all, the whole point of adapting something to a new medium is to expose it to a new audience. You want to make something that can be enjoyed by people who don't even know it's adapted from something else, or who can't tell one way or the other. You want it to satisfy people for whom it's their first exposure to the work, so that they're judging it purely on its own terms rather than constantly comparing it to something else. Ideally, even people who do know the original should be engaged enough that they just let themselves experience the work in the here and now, rather than distracting themselves with comparisons to something else.