That's because Metapod is cocoon. Cocoons are supposed to look completely different from the final form. Dragonair and Graveler are not cocoons. Btw, Dragonair and Dragonite are completely different. Dragonair is a serpent, Dragonite is a chubby dinosaur with some tiny wings. They have completely different color schemes too.
Magikarp should have evolved into Dragonite and Dragonair should have evolved into Gyarados. At least, this would have made more sense aesthetically.
Games and debauchery.