To those who don't like the MC acting as a kid despite her mental age being college level

it's not as if she wants to act like a kid, she just can't help it.

She was an adult in her past life, but since she's a kid and everyone treats her like so, she'll unconsciously identify herself as a kid and therefore act like a kid.

It's like the Stanford prison experiment (or the psychological prison experiment if the name's wrong) where the guards (that were just normal civilians that were given the title 'guard') became too engrossed in their roles that they soon identified themselves as guards. Same thing with the prisoners.

^ if you're too engrossed in the role you were given (plus adding external factors such as your environment) you will unconsciously take on that role and act like it. It's like second nature to her to act like a child despite being an adult.

And there ends my TedTalk. Kudos to whoever finishes this.