To be fair, she transmigrated into a child's body, she's physically a child so her brain is also that of a child meaning it's not that developed.
And since we don't actually know what it's like to transform into a child in reality, we won't actually know if an adult's going to retain an adult mental capacity when they're suddenly in a child's body.
Writers either make these characters retain their adult minds or have these adults adjust their mental capacity to that of a child's.
This story is obviously the latter, so can you really fault the fl for being "stupid" when she's suddenly in a body of basically a toddler?

Wtf fl is really stupid when she was in military how did she survived with this much stupidity