I think she played the role too mentally challenged - the creative team decided to go in a different direction. A problem with the show is balancing Clara's character so she's not TOO slow, otherwise the relationship with Fabrizio turns really creepy, really fast. Also, they decided to make Margaret keeping Clara's problem a secret a main plot point that is only revealed to the audience in the middle of Act II, so I guess Celia made that secret a little too obvious, while Kelli just seemed like an immature, naive girl who doesn't think before taking actions (at least until Hysteria).
"Sticks and stones, sister. Here, have a Valium." - Patti LuPone, a Memoir