Everyone can do what they want with their life... but this is my opinion so: I think that unless you are in an inevitably irreparable situation(that there is no solution or that the solution is very far from your control), CTB is a cowardly option.
I think it can definitely be cowardly, but I wouldn't say all suicides that outside what you described are cowardly.
Some people may not be in excruciating pain or anything, but they would just prefer not to be alive anymore because they don't take enough satisfaction from it and they've tried to make it better but nothing seems to work.
I know you might say that falls into the category of irreparable situation, but I assume you mean situations that evoke a certain severity of suffering.
What would think of someone who has, for example, moderate depression for 10+ years. They're not in extreme pain, they can function, but life simply just doesn't offer enough satisfaction, and no professional help will work.
Another e.g. I've seen a lot of examples of Schizoids who killed themselves. Their condition brings with it anhedonia that doesn't cause them to suffer much but makes it so little if anything gives them satisfaction, and there really isn't any treatment.
Their decision to kill themselves is a rational decision. They simply would prefer not to live.
So would you consider it cowardly for a person to CTB, not because life is too painful, but simply because not being alive is a preference to some people?