Yeah, people have unrealistic expectations for character development when you see a lot of people irl that are still mentally 12 at 65 years old.
The people who want to read about someone stubbornly refusing to grow up are few and far between. They may be "realistic," but realism is often a trap in storytelling because reality tends to be sort of crap. What most people want is "believable," not "realistic." Immature older characters are almost exclusively used as negative foils or comedic anti-role models because of this inherent subversion of the maturity expected with their age.
That is not the problem with Chita, nor why many readers find her to be an irritant. Rather, it's due to her being a fairly static character so far and repeatedly being used for a "break" from all the coming-of-age developments that are the main meat of this series. Much has happened around her, but very little has happened TO her. Static characters in coming-of-age stories are usually adults who have already "grown up," or youngsters portrayed negatively because they refuse to "grow up." Chita is neither. Thus, like a bump on an uneven surface that refuses to go away even as the rest of it is smoothed out, she becomes increasingly irritating to many readers with each repeated appearance.
Her character arc has not been handled well so far, but I pray that with the dedicated attention an arc focused on her brings, the author will be able to salvage it.