Keats' opinion of art is paradoxical and multi-layered.
There are many levels to art: the art piece itself, the depiction on the art piece, and the viewer. Keats seems to be saying that the urn is affected by time differenlty than the viewers: whereas the urn ages slowly (its carved lines fade), the viewers age in real time (grow old). Obviously, the urn has and will outlive most of its viewers. It is, in a way, frozen in time.
Paradoxically, the action on the urn is in motion. It has its own "time" and "movement." Since an urn is a 3-dimensional object, and since it has a wrap-around type of narration, the urn functions as an early form of "moving image." The viewer must walk around it and can never see all of it at once.
Much hinges on who says the last two lines:
'Beauty is truth, truth beauty,--that is all / Ye need to know on earth, and all ye need to know.'
Is it Keats speaking here? A persona? The urn? Do the lines mean anything at all?
Dr. Milani (see source below), a CUNY Brooklyn professor, says this:
Some critics feel that Keats is saying that Art is superior to Nature. Is Keats thinking or feeling or talking about the urn only as a work of art? Your reading on this issue will be affected by your decision about who is speaking.
In other words, Keats paradoxically leaves his opinion of art up to us.
It is indeed a paradoxical vision of art that emerges from Keats's Ode on a Grecian Urn. The paradox is a still older Keatsian one where the only choice given to man is between static eternity and dynamic mortality. The Urn, representing the virtuosity of plastic art is something that has arrested motion into a stasis that is perpetual. The sacrificial journey of faith or the sweet pursuit of a beloved kiss have all stopped on the verge. While these images are preserved by art in the changing flux of time, they are also stripped of their natural culmination. They are thus the 'cold pastoral' of the frustrated odds and ends. There are implications of escapism and an unfaithful idealization of life in art.
I think, the most radical and dangerous implication comes in the fourth stanza where Keats goes beyond the mimetic character of art to talk about a process of transference. The objects are not duplicated in the world of art but are taken out of the real world and pushed into the immobile immortality of the artistic universe. Thus, says the poet that the sacrificial journey-makers with their mystical priest are permanently captured by the urn and the little village they belong to would always remain deserted therefore.