I thought so too. She's probably sharper than she appears, that Utsuho is.
Also, a pretty cute story. ^^
Well, not just anyone can count ten years in days to such exactness. Okuu always strike me as an odd genius.
For Okuu, her perceived dullness is what allowed her to control fusion (a very complicated power, one that the universe thrives on). So perhaps her "bird-brained" emptiness is not a lack of knowledge or intelligence, but rather an indication of her ability to embrace the truth whole-heartedly. Unrestrained by unnecessity.
In a more conventional sense, her pure-hearted innocence gives her the responsibility to handle such a destructive power. It is the most karmatic form of justice; a power bestowed to someone worthy of handling it. Especially after Subterranean Animism, as she seems content to stay empty as she is. Not because she has no pursuit for intelligence, but because she realizes that she harbors a great power and understands the consequence for its misuse.
So Okuu can't really be a bird-brain if her perception of duty runs so deep, and her perception of destiny so pure. She's awesome...
