From Bulbapedia, the Clefairy article;
Clefairy was originally going to be the official mascot of Pokémon, but Pikachu's continuing popularity caused it to be chosen instead.
However, it's not like I can verify this is true, but it doesn't say anything on Jiggly's page. :| And why would you think that after they made the executive decision to make Pikachu the mascot, they would consider Jiggly? Jiggly was a lot later in the anime. :s This would have to be a decision made BEFORE the anime had started, not afterwards. Jigglypuff might signify their consideration for it, perhaps. *Shrugs*