Someone posed a question today: why do some Pokemon wear clothes? I think we're addressing a biological/environmental/social concept here.
Some Pokemon's genetic material produces structures that may appear artificial but are simply a natural element (consider the markings on butterflies). Other Pokemon are conditioned by necessity to improve themselves through the adaptation of items found in their environment, while still others, either out of social acceptance or mere personal interest, acquire accouterments not natural to their person.
Biological example: perhaps the Meowth's and Persian's forehead "jewelry" is analogous to the pearls discovered in clams.
Blastoise is a great example for environmental conditioning, as that particular species has discovered that cylindrical human refuse (pipes, tennis ball canisters, old potato cannons) is a superior conductor of the water ducts that occur naturally beneath the carapace.
As for social expectations, consider that prolonged exposure to humans has impressed upon the Machamp and Jinx the virtues of modesty, while the combat minded Primape discovered the advantages of training with weighted cuffs. Kadabra and Alakazam benefit from natural intelligence combined with remarkable psychic powers, and perhaps at some point in the past their ancestors discovered that the Spoon is a universally recognized icon representative of their abilities.
