Tezuka-sensei's style is very round and cute, that's all, so his characters look a lot younger than they are. Compare her with Astro! Just by looking at the design, people would assume toddler, but in-universe, she looks 7-8!
But also, you're right that her body does look doll-like. I like when Tezuka-sensei draws her face more maturely, like an adult's, while her body stays doll-like. It's not very consistent, she had loooong legs and bigger head at the start (she reminded me of those Asian plush dolls with the long legs, fff), but looked more equally proportioned in later designs.
I like the more streamlined and proportionate PInoko from later chapters! Compact, ffff.
Also, she DOES have a doll-like body. Her prosthetics are mostly made of resin!
Sometimes, a bit like Astro in the 1980's anime, you can see thiiin lines where her joints connect, like in this flashback.
A stage play has had her depicted with a doll-like robot, the Takarazuka play with an adult actress, and The Visitor in the Eye film with a child actress (but also, Pinoko is a set of twins??? I haven't seen that one yet, I don't like the idea of a child playing Pinoko personally, squicks me out.)
So, if Pinoko looks a little too perfect and doll-like... good! She's supposed to look a little strange, her body isn't a natural one, it's largely prosthetic, and comes with its own challenges. In the anime, it seems she just doesn't know how to (as far as I recall), but the manga makes clear her mostly artificial body is too heavy to float. We see this in Move, Solomon!
She sinks like a rock in most water, unless it has dangerously high levels of salt, in which case, she can only swim for a little bit, after which, she should be quickly washed, so it doesn't rub her skin raw. We see this in Water and the Badness.
Pinoko and Black Jack are both disabled leads and that's part of what makes them so complementary. ^^ That they look a little strange or off-putting to people is fitting, then, even if people can't always tell Black Jack was disabled growing up because he's so healthy now (after a loooong and excruciating rehabilitation), but his scars and skin graft always turn heads.
