I would think there are broadly two ways to treat this kind of well-known simple moral tale in a picture book. You could: use it as the springboard for some virtuoso illustration, playing with character and setting; or elect to let the words largely speak for themselves. Imai takes the second route, providing carefully composed illustrations of delicacy, restraint and some quiet humour.
While these illustrations offer little in the way of characterisation of the mice or dramatisation of a theme of the contrasting merits of simplicity and sophistication (except in showing the sophistication of art in the service of simple storytelling), they are distinctive and elegant, provide pleasure in themselves, and subtly invite the reader to make their own choice of mouse life-style. While the text favours the view of the country mouse, the penultimate illustration of the two mice each taking their own road to happiness is rather more even handed.