Celebrity casting can work. Sometimes though a celebrity is just there because they needed a name. Sia was pretty pointless in the mlp movie. That’s the biggest problem I have is when a celebrity is just shoved in because they wanted another name on the poster.
Or then there’s situations like Sinbad: Legend of the Seven Seas. Brad Pitt did the titular Sinbad’s voice. He does alright. He’s fine. It works. But he didn’t stand out or anything. They could’ve easily used a normal voice actor. Actually, they almost did, a tv series was planned to follow the movie and Brad Pitt would’ve been most likely recast.