The speaker overhears their tailor singing and guesses, with a mix of amusement and tenderness, that he is trying on a dress. The poem never says this outright. It gives us a quick, almost gossiping glimpse of a private moment: a man alone in his closet, enjoying himself.
The name "Mr. Thin" and the phrase "a pretty man" make him seem delicate, a little theatrical, someone whose public self may be more restrained than the one inside the closet. The speaker doesn't mock him. The tone is light and curious, even affectionate.
The last lines turn quieter. He is "weary of meeting nobody there / but a moth." That suggests loneliness: he has a hidden side, but no one to share it with. The moth is a nice touch, since it is a creature that eats fabric, which is exactly what a tailor works with. The mood moves from playful to gently sad, leaving us with a small, odd, human portrait.