You've got to feel sorry for this poor young (wait...) girl. After all she's been through, she really deserves a break. And being the most brilliant mind in the Universe doesn't help out her little stress problem. She had been locked away in a dimension created by Kagato for many years (thousands, I think.), and the only reason she ever got out was because of Mihoshi's little careless mishaps. She takes things very calmly, even when it gets their worst.
But why "Little Washu"?
In Episode #8 of the OAV Series, it is revealed that she had a son at
one time, taken away from her because her husband was of higher status
and was not allowed to be married to her. There was nothing she could
do about it. From then on out, she wondered one thing that is puzzling
to even the greatest of all minds...
"Liniage... Birthrights... What does it all matter?
That's what all the adults think about. I'd rather stay a little
kid than to be a grown-up and have to think about all that ignorant, small
stuff that doesn't matter."