Jigglypuff no doubt got in because of the show. However, Pikachu, Mewtwo and Lucario are popular even outside the show, and Pichu was regularly used for promotion during Gen II, and it was easy to make a Pikachu clone and joke character at the same time. Mewtwo was the strongest Pokémon until God decided to join in on the fun in Gen IV, and Lucario was the first Gen IV Pokémon to be revealed outside the starters, which made him more popular than any other Gen IV Pokémon, which was the current generation when Brawl came out. Squirtle, Ivysaur and Charizard, aka Pokémon Trainer, are obviously based 100% off the games, and frankly I'm surprised Charizard didn't make it into Melee given that he's without a doubt the fan-favorite Pokémon.