Whoever misunderstood and took a pity on heroine, please don't misunderstand anymore
She's definitely person with same condition as Bertia, a reincarnated person into the game. She kept saying emotionless doll prince and third rate villainess while in fact Bertia wasn't evil in that world, very different from the game. She's a lunatic that forcing Bertia supposed to be evil and when she's not, this so called heroine called Bertia a third rate villainess. Just like Prince Cecil said, both were too focused on the plot while one tried to make others happy, one tried to be 100% heroine like in the game (chuunibyou I said and a very selfish one) to the point she actually thought it's only a game and none all of them were real people and so she didn't care about their own feeling but her solely goal such as reverse harem and princess (future queen). In addition she didn't love the prince instead she thought the prince should be grateful that she picked him as the ML so he wouldn't meet his doom day.
Tldr: her expression on the last page, I doubt she would be sad that Pii was gone, she's probably too furious and would blame other such as Bertia or Prince Cecil for killing her bird (and how the bird died in vain, didm't bring any benefit for her)
Althouth honestly, bird being bird brain, blindly swallow heroine's words that didn't match with the 'reality' (not game). Honestly it's better if she's sadden by Pii's gone (have no power to form as spirit anymore) if not, imagine Pii saw his beloved heroine in the end only giving cold shoulder for his dead lolololol