I'm leaning more toward the second option, as well. I suspect Nagandel's vision of the flowery field is what Ultra Megopolis (or whatever the anime decides to call Poipole's world) was like before the light was sucked out of it. If that's the case, then it stands to reason that the lack of light prevents anything of use from growing, including Berries. I don't think Berries will be enough to heal Nagandel, though.
There's also the meteor in its vision. If the lack of light didn't kill the plants, I wouldn't be surprised if the meteor was the cause and the lack of light is just preventing things from growing back.