Like others have said, the answer boils down to: "it depends". It's a very complex question, and the answer is not black and white.
If someone is in a situation where they cannot improve their own life, whether it is due to the circumstances of their life (like a terminal illness) or the effort required to do so is so monumental that someone in such a low, depressed state could not even begin to put forth that kind of effort or determination to do so, then perhaps rationally we can call it a good thing that a sentient being is no longer suffering.
However, if someone decides to CTB on an impulse, when in reality their situation could improve, they have the energy and will to do so, and they have not exhausted all their options, then yes it is a good thing because it gives that person a second chance to turn their life around.
Obviously, many different factors play into whether a life is repairable or not, and many times even the suicidal person, who knows themselves the best, cannot determine the answer to that, so there's no way we could without really knowing the life of the person in question.