>>51291595Before Delta Species, there was Crystal Pokémon during the e-Card series back in Gen 2, which may have led to that design choice for Terastal as both involve crystals with Crystal Pokémon's Crystal Type ability and Delta Species during EX Crystal Guardians.
I don't mind the crystallized look, but the crowns could've been done better. I get that they're there to easily showcase the changed type, but they didn't have to be so big. Others have pointed out that there's also potential issues with the crowns on Pokémon like Hitmontop.