So I had a big spiel but I backspaced when I realized: hey is it weird how little firm sources are on the Clefairy mascot thing?
Bulbapedia references (with no actual snippets) a Becket magazine saying that Clefairy was in consideration but went to Pikachu because of the anime & Pikachu's general familiarity. Which, without seeing the actual magazine, kind of seems like that might have been from the western arm so that feels like context is missing
And then follows up with a COMPLETELY unsubstantiated claim that Clefairy was going to be in the original pokemon for Ash but was changed at the "last minute" which gonna need some citations on that please
Even checking a few other articles that happen to come up mostly eschews sources and just builds off the original concept, and then presents their own theories without really mentioning who thought what and when.
Hmm - yeah, you're right. I'd always thought the developers admitted it, but it does just seem to be that one source. The only other evidence is that manga where Red has Clefairy as his starting Pokemon, but that doesn't necessarily mean anything.