No matter what anyone thinks, canon is official, and Kopa simply is not. I get quite tired of reminding people of this. Disney themselves did not write the books he was in, nor did they write the books and comics the other semi-canon characters and cubs were in. The ONLY TLK stuff that is canon is what comes from the movies and the filmmakers... sorry to burst your bubble. "Fluffy" is Kiara, not Kopa.
Not that I necessarily have anything against Kopa or any of the other semi-canons, but they are NOT OFFICIAL and NOT IN THE MOVIES.
As GeminiGemelo points out, it made sense to use a Simba-clone in wrapping up the first movie, but not so much when said clone got their own movie. Kiara couldn't look too different from Simba BECAUSE Fluffy already had looked so much like him, but at the same time, she needed to be unique, so the design became what it is. I'm glad they changed her design a bit on the one hand, because she doesn't look exactly like Simba, but sometimes I wish they had left her as a Simba clone so these deluded people wouldn't be able to use her looking different as "proof" that Fluffy is Kopa.
Disney has said that Fluffy is NOT Kopa. There's no debate about that.