I do like the rarity system in Elements, it's clean and simple, usually isn't game breaking, and doesn't grab for money.
That said, rarity in other games is almost always a marketing tool. It's fantastic that this game is free, but I don't see why the chances of getting a nymph is so low. Increasing it to 10% isn't going to get anyone 72 nymphs (or 144 if you want sets of upgraded ones) any time soon. Assuming that you luckily never get a 7th nymph of an element, getting 6 of each element would take ~2 years of daily Oracle spins at a 10% nymph rate. I'm not particularly concerned unless nymphs ever become staples in top decks, though.