I think we can all safely agree that Jan Pinkova’s character designs for the film are spectacular, but she withdrew with the film, putting all responsibility on Brad Bird. My questions are A) Why did Pinkova withdraw from the film, and B) How did Bird feel about taking over the film? The reason why I ask the latter is because I figured he would have an attitude about it since it wasn’t one of his original ideas.
I believe this thread can help you with question A (it’s about why Jan left Pixar, but there’s also some information about him being replaced as director of Ratatouille). For question B, here’s a quote from this article.