While I understand that materials in this game have different variants, "cotton" that is reportedly on these tundra islands is so rare that in roughly 6 explorations of tundra islands in search of fiber-bearing plants I found none. It's for that reason I came to the forums to submit a bug report about it. Fiber is far too important a resource for it to be so rare, especially when you can spawn in these regions. Altering the rarity of higher-tier materials like metal is certainly sensible, but fiber? What makes it even more confusing is that the plants on the starter zones in this biome have fiber in these plants, but when you go to the other islands those same plants no longer have any in them. From a variety of perspectives it just doesn't make sense to me why fiber would be omitted as a common resource in these areas while wood, stone, flint, thatch, animals, berries, etc. are just as plentiful as anywhere else. Especially with the tundra biome being as unforgiving as it is for new players due to temperatures, further reducing the ability for players to survive makes the area feel more off limits than challenging.
Right now a lot of people seem to think that there isn't any fiber on these islands at all. The pervasiveness of this belief says to me that if nothing else, it's too rare.