Materials have been pretty random from what I've seen. I tend to do most of my material collecting on High Metal Content planets, but honestly, the drops are about the same as they are on everything else, even Ice worlds.
That is something I'd like to see them improve. Grinding should never be a gameplay goal, especially for consumables. I spend enough time on exploration, I don't want to have to stop exploring so I can run around the surface of a planet for four hours until I find the one rock I'm looking for.