>>37102229Ditto is easy to explain because it makes for a simple marbit that's still recognizable even if it's messed up. The top of Poliwhirl (eyes and arms) is actually the bottom half of Ditto flipped upside down. Oddish and Poliwhirl have the same legs so that's even less work than having to make a new mold for each pokemon. Oddish's spikes remind me of a certain icing nozzle so I think that's how they decided to include it. And then they did Pikachu separately because he's the mascot.
The three of them cleverly reused the same cutters, as opposed to having to make different cutters for bulbasaur, squirtle, and charmander.