> So there is a chance “that’s beautiful” means that the person showing him the painting is beautiful?
No, but I think the choice of adjective is important.
Suppose instead of saying "That's a beautiful painting" I said "That's a masterful painting".
The sentence structure is the same. The adjective is still modifying "painting". I'm still saying the painting is good. But the word I chose more directly implies a judgement of the artist: I am describing the painting as the product of a master painter.
I get the same impression from hearing "stupid idea". Stupid is a description of intelligence, thinking ability, aptitude: attributes which lie with the idea's originator, not with the idea itself.
