The speaker plays a snobbish gallery-goer, complaining about the "Plaster People" who stumble among the "true Art" and leave powdery footprints on the good carpet. They smudge everything "canonical and clear." But the poem keeps undercutting that voice. The dust they scatter is "the stuff of stars," and the people themselves rise "up from the foundations," as if they are what everything else is built on. They have "buried / every civilization in turn," which suggests they are older and more lasting than any museum.
The last line, "Oh, look on their creative / struction, and despair!", twists a famous line about ruined power. Judging by that echo and the odd word "struction," the poem hints that these messy crowds are the real makers, while the tidy art world only guards what is already finished. The mood is mock-indignant and funny at first, then quietly awed. The speaker sounds like they are complaining, but the imagery sides with the dust.