He wouldn't be okay. Katniss had seen it on the page, in her own words: Peeta was irretrievable. He had been poisoned, in body and in mind, and he couldn't possibly be okay ever again.
She couldn't take much hope from what Finnick was saying. If she had discovered this back home she would have done precisely what she'd done there-- run away from it as far as she could, into the fighting, to try to have an effect where she could. There was nothing she could do for Peeta besides send him into terrified and murderous rages. And from here, she couldn't even do that.
But she could do something for Finnick, still. "Maybe," she said, which was as much of a positive response to his words as she could muster. His thoughts did count for something, although she thought that optimism was better geared toward his own life. Maybe that was harder than being hopeful for her. Finally realizing what she could say that would actually be useful to him, she echoed his own words back. "I think Annie and Johanna will be okay, too."
For whatever her thoughts were worth to him, at least that one she could genuinely feel hopeful about. Annie had seemed okay, so maybe they'd left her alone in the interest of torturing Peeta and Johanna. Or maybe they'd simply known that for the poor mad girl, being alone in her mind might be worse. Especially if she had any idea of what was happening to her fellow captives. Regardless, she'd been happy to see Finnick, and wasn't that really all that could be hoped for at this point? Whatever had happened to her, she could overcome it with his help.
But there was no way to know for sure, so maybe he'd needed Katniss to say it. That was probably their best bet for survival here: being optimistic for each others' sake.