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I kinda agree.
From the small clips we saw and how popular her show was said to be, she did have talent. However, I bet anything that her talent was stunted. From what we've heard she only had one real role: Baby-Doll. With her condition she likely stuck to kid-based acting, so anything outside of that was likely difficult or off in her performance, not as good as her TV show character.
Naturally no studio wanted to hire her, so her acting skills for more mature characters couldn't grow by much besides what she herself practiced. That times her appearance made audiences and Hollywood turned off, I also suspect that Macbeth was just a trashy movie to begin but sense Mary played a lead she got most of the blame. By the time she realized she couldn't get hired and begged to have her show back the world had moved on.
Another small thing that really pisses me off in regards to Baby Doll is the ten seconds in the episode where it shows her playing the role of Lady Macbeth. From what we got to see, even if it was less than a minute, it was a pretty decent performance and didn’t really deserve Robin’s negative comment on it. I may be reading too much into it but it really seemed like she was a decent actress and people just refused to take her seriously because of her condition.