T!mmy is essentially right here...
The mirror essentially takes what evil lies within you and amplifies it into a tangible mass, molding it into an evil double. Metaknight, while not evil by any stretch of imagination, certainly has much pain in his past and has internalized a lot of angst and sorrow. This abundence of dark energy within him was utilized to create a very much evil version of himself. Kirby, on the other hand, is famous for his level of innocence and inherent goodness, much like Mario. Like T!mmy said, there simply isn't enough negativity in Kirby to generate a sufficiently dark version of himself, and thus his reflection comes out more odd than evil.