What started ruining his credibility IMO is when he leaned towards teams should do what he thinks. He mocks teams for picking players outside of his rankings. If he would just stick to saying things like "Ok, I had that player lower in my rankings based on personal research and what I've heard from teams, but obviously they like what they see". Instead he interjects disgust.
I guess that's fine when your rankings are more right then wrong. Because Kiper is so wrong so often - he lives in the world he created. Now he's having to deal with not only ESPN bringing in others to combat him live as they laugh at him - he subjects himself to widespread criticism from competing networks, fans, media, NFL alumni and NFL teams themselves.