Well, I think it depends on the Animorph, really. (I'm going to ignore Megamorphs 4, and pretend that they would have had the chance to live normal lives had they not been Animorphs . . . . and I'm also ignoring the fact that there would have been no book series, and only considering what would have been better for the Animorphs themselves).
Tobias would never have been happy. He may have eventually committed suicide, but even if he didn't, he'd have lived a miserable life as a normal kid. For him, I vote no.
Rachel was a lot better off before she discovered the darkness inside her. That's something you can never un-learn. And without the war, she'd have been perfectly content in her ignorance, and would have just kept on shopping and doing gymnastics like she'd done before. For her, I vote yes.
Jake was like Rachel in a lot of ways. He sort of discovered a darker side of himself, too. And its obvious that the war changed him for the worse. For him, I vote yes.
Marco . . . . he's easily the most adaptive of the Animorphs. He complains a lot, sure, but he can handle almost anything that gets thrown at him. He made it through the war okay, but he would have done fine without the war, too. For him, I vote unsure.
Cassie. She's . . . a tough one to say, actually. She hated the war, that much was obvious, but was it ultimately good for her as a person? I think so. I might even say that she needed it. She was so naive before, and even during most of the war. She just saw the morally right way of doing things, and never the necessary way. If she had lived a normal life, with an attitude like that, and as polarized in her beliefs as she is, she would have gotten steamrolled when she finally hit the real world. Good guys finish last, as they say. I mean, it took a freaking war to make her see that her view of morality isn't all there is. And even then, I'm not sure the message really stuck (she seemed to get it in parts of the final arc, but in book 54 she seemed to go right back to being her old self). So, for her? I guess I'm still voting unsure. The war was bad for her in the short-term, but possibly good for her in the long-term. It's fine to be nice, and to hold onto your morals, but you need to be able to at least see beyond them.