i'd probably say it's the other way around...
But neither of them are dumb either

actually you just want to be nice to papyrus because you have a decent mentality and it would feel bad to tell the truth

papyrus is not the brightest bulb in the box

No, i actually think that.
Just because he isn't the brightest bulb in the box doesn't make him an idiot

you could say he aint the sharpest tool in the shed