Not so much when she's possessed and all. And I don't know if the evolution is really entirely realistic... it mostly worked for me, though. Lots of people are self-obsessed or narcissistic in puberty but grow out of it later. And with Cordelia it's partially an act from the start, as we find out quite explicitly at some point in Buffy. Turning her into a higher being was a bit silly, but then that turned out to be fake anyway, so well.