Three short lines, and a whole small scene. A cat and snow are the only characters, and the poem says they were "merry" all day, left to themselves. The title, "Inside and out," hints at a divide: someone is indoors, watching, while the cat and the snow carry on in their own world. Judging by the title, the speaker may be inside, looking out, though the poem never says so directly.
The key phrase is "by themselves." It suggests the speaker isn't part of the fun, and that may be the point. There's no complaint here, though. The speaker simply notices and enjoys how cheerful the two seem without any help from people. Calling snow "merry" is a playful touch, as if it were alive and in a good mood.
The mood is light, quiet, and a little amused. It's the feeling of watching something simple and happy from a warm spot, and being content just to witness it.