Everything would be different. Gags including characters or bikes getting shocked would no longer exist, unless Clefairy would also know electric-type moves or randomly metronome electric attacks a lot. In fact anything that involves electricity would probably be non-existent or taken over by another Pokémon.
A lot of gym battles would have been different. Lt. Surge's for one thing would have been totally different unless Ash were to catch another Pikachu later. And speaking of Surge's battle, I wonder if there would be some situation in which his Clefairy would refuse a Moon Stone to prevent it's evolution.
As for marketing, I do think that Clefairy may or may not have been as popular with young boys unless they gave it a good personality that would appeal to them. Maybe, and hopefully, not like the Pocket Monsters manga Clefairy. In fact maybe it could have Pikachu's personality and still appeal to young boys. But I'm glad we got Pikachu since I just find it more marketable to both genders.