Obviously Neelix literally could cook, because we saw him do it. He looked like he knew what he was doing. On Talax, he could have been considered a master chef for all we know.
But not everyone is going to like his food. That doesn't mean he's a bad cook or anything, it just means that the crew doesn't like Talaxian cuisine (assuming that's what he was making).
Replicators have spoiled them because they've been able to eat whatever they want exactly to their taste and now suddenly it's out of their control.