I think she just have a different standard of happiness. A lot of isekai FLs be like, "I have to be stronger," or "I'll open business so I can get rich," or "I have to figure things out so I don't die."
She wasn't reborn as a villainness or a noble side character. She was reborn as a slave, living in countryside. For her to live peacefully is enough, she doesn't need to get the spotlight. When she was a princess, she's undoubtedly unhappy.
My point is the reason why she looks dumb was probably because she doesn't have any expectation or major goal, just living well and happily. She doesn't assume anything. Which I understand is very different from typical protagonists.
I'm kinda not loving the way she is pretty dense and dumb...
No offense tho!