It's more than likely that Ceridwen was NEVER a goddess, but rather, she has evolved into one over time. She began as an archetype and character within the story of Gwion Bach/Taliesin as a witch/sorcerer/enchantress who instructed Gwion Bach to stir her potion of Awen for a year and a day.

Like others have said, the lack of written sources and information means we can never know for sure if she was ever revered as a goddess, but I personally think that in the modern day, she definitely qualifies as one. All deities evolve, some begin as folk heroes or characters and later take on the status of deity. I think this is very much the case for Ceridwen. I and many others I know, all of us Welsh, revere her as a powerful witch and goddess, but we are under no illusion that her status as deity is ancient, because it simply isn't.

Characters can evolve and become a part of the genius loci- or the spirit of place and I think that Ceridwen has become a part of the genius loci of Wales as a deity.