To the same extent that every trait is relative, I suppose. There’s not a big physical block of stupid somewhere we can point to, so we have to judge it relative to others. If someone is kinder than most people we say that they’re kind, if someone is smarter than most people we say they’re smart, if someone is more beautiful than most people we say they’re beautiful - it’s not something unique to stupidity or to negative traits. It ‘doesn’t exist’ in that it’s not something you can pick up and hold in your hand, but it can be measured as an identifying trait when compared to other people and generally agreed upon.

I could say my dog is brown and my friend’s dog is white. I can’t go touch ‘brown,’ it exists only in my mind and my brain’s perception of wavelengths of light, but the brown dog is still more brown than the white dog. As long as you have 2 objects you can come up with these kinds of differentiated descriptions of them. (I wouldn’t say “my dog exists in 3-dimensional space and breathes air” because all dogs exist in 3-dimensional space and breathe air, therefore it isn’t usefully descriptive the way “brown” is.)