The speaker admits to a small, slightly guilty pleasure: the word "pretty" is a lie, yet it's one they love to translate. The poem seems to be about the shortcuts we take with language. "Pretty" is vague enough to cover almost anything, so it works as "a ready key to forms," a single word that unlocks or labels many different things without really describing any of them.
The last two stanzas turn this into something sharper. Calling something pretty is "the best of all plaudits / at writing off." The phrasing is neat: praise and dismissal become the same act. To call a poem or a face or a landscape pretty is to compliment it and also to stop looking at it. The speaker seems to understand this and enjoy it anyway.
The mood is dry, playful, and a little sly. There is no anger, just a wry honesty about how easily a kind word can close a door. The short lines and compact wording make the poem feel as quick and neat as the word it examines.