Also, her hair doesn't stay white, it turns back to it's (natural?) color by the time she gets on the bus.
But no, she's no goddess. I mean, she said 'Oh My Goddess', not 'Oh Me Goddess'. And Kennedy's opinion, whether a figure of speech (which it was imo) or a statement of perception can hardly be taken seriously. What would Kennedy know? She's just a silly girl with new slayer powers, not an oracle.
So no, Willow is no goddess. She's just the Uber-Super-Duper witch we've all come to know and love. The only difference, is she tapped into good magic for the first time.