Duolingo should post pictures when coming up with sentences like this one! How does a stupid mailbox look like?!
I'm confused by the last two words--does it mean it looks dumb (outside) or is 'ut' part of dum?
å se dum ut = to look/appear stupid
You'll see this structure with all kinds of adjectives that can describe either the appearance -or- some other quality of a person or thing, used to clarify what is meant.
"Han ser dum ut, men han er den smarteste personen jeg kjenner."
"He looks stupid, but he's the smartest person I know."