Was Aerith always this "simple"? By simple I mean she acts like she's 5 yrs old. It's quite jarring when compared to other characters that seem to have more depth or at least comedic value.
Yes and no.
She's certainly more happy go lucky than the rest, which is a welcome break from the general anguish of the other characters. But her character has a deep, old wisdom, she's supposed to be representing someone with a deeper connection to the world but also someone who is "the best of them".
Hence the impact of her death.