Are there actually adults like this in real life? That don't know how pregnancies work? I've seen it enough times in manga that I know it's a trope of innocence, but it's absolutely ridiculous.
Outside of extreme/cultural/sad circumstances, no. People are ignorant about various parts of pregnancy, but they know at least how it happens. As you said it’s a trope, only her appearance is supposed to be childish, so not knowing things she should at her age is the joke.