Sure, triple green is somewhat of a restrictive mana cost, but mana bases have never been better and neither has access to fixing
"Any target that isn't a Dinosaur" means any non-Dinosaur creature, any planeswalker or battle, or any player (no matter how old they are)
The difference is just in waterbending 10, which isnt the easiest thing to do
Like most Magic: The Gathering cards, the value of Magma Jet depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Modern Masters 2017)
Cards like Mycosynth Lattice or Trinisphere push Karn from irritating to game-ending, and the fact that this happens without attacking explains why he ranks among good planeswalkers, arguably one of the best to ever get printed for artifact-heavy metas