The speaker admits it's hard to find anything worth looking at in "the art of the foolish." The title, "Artless," plays on two meanings: art that lacks skill, and people who lack any pretense or craft. The poem seems to be poking at both.
The "foolish" here are content to leave the canvas "as is," untouched, as if the blank surface were already enough. They don't invent or imagine anything beyond themselves. The last line is the sharpest: they are "making up nobody but themselves." Judging by the wording, that could mean their art only ever reflects their own ego, or that they never create a real character or world outside their own small view.
The tone is dry and a little scornful, though quiet rather than angry. The short lines and lack of decoration mimic the emptiness being described. The mood is one of mild impatience with laziness passing itself off as art, and the poem leaves the reader to decide how fair that judgment is.