> Do you subscribe to the view that effort is what makes art valuable?
Not as a hard rule, no, although there is absolutely art where the effort to produce it is an inherent part of the value of it.
> Do you think that the matchstick cathedral is more artistic than the portrait?
No, not because of effort involved, anyway. If both things are trying to convey something about the human experience, they're both equally "artistic".
I subscribe to the view that what makes art valuable is that it communicates something human that can't be communicated well otherwise[1].
This is why if there existed an AI that produced art all by itself, I would consider the work produced to be pretty pictures, not art. An artist using AI as a tool to produce human communication, though? Totally art.
> (Which is a different question from what makes art have monetary value.)
I agree. The price tag of a thing says nothing about its artistic value. It only speaks to commercial value.
[1] This is also why I don't think there is any such thing as "bad art". Art can be successful or unsuccessful, but "bad" and "good" are subjective aesthetic judgements. When people say art is "bad", what they mean is it's art they dislike.