None of the plants or seeds found by the Sniffer have been marked as Uncommon, despite the Sniffer Egg now being Uncommon and being required in order to acquire these seeds. This seems to go against the guidelines set in place for rarity:

• The chance of finding it in a loot table, including mob drops
• The amount of travelling required to obtain it
• The difficulty of any obstacle which must be overcome to obtain it
• The possible amount of that item or block which exists in the world

Since a Sniffer Egg is required to get these plants and seeds, they should be the same rarity.