This Tier List is Bottom Tier.
@Basara, Jigglypuff has been a very well known Pokemon from the earliest days of its popularity, and remains even today a staple of the franchise, comparable even to other still-important original Pokemon like Meowth or even Pikachu. Considering as well, that a large portion of the gaming world did not bother to keep up with anything more than the first installments of the Pokemon franchise, most characters chosen are from the original cast, not surprisingly. This was, in fact, a rule up until Lucario's appearance. In pokeballs, some greater freedom is allowed, and newer pokemon appear in droves inside Brawl's Pokeballs.
But playable Pokemon should be well known and should have appeared relatively early in the franchise (excepting Lucario's dubious, though understandable selection). All things considered, Jigglypuff's presence is no surprise, especially when coupled with her position as one of only twelve in the original cast, survived through Melee and into Brawl. At the time of SSB64's creation, there were few more well-known or popular choices than Jigglypuff, and from that point on, her inclusion in the original cast has nearly guaranteed her continued presence.