Transformation in The Witches is a very literal theme. That means that it's not about transforming something inside of you, changing yourself for the better, or any of that. It's about actual physical transformation. The witches like to change little boys and girls into other things: oil painting figures, chickens, slugs, porpoises, stone, and, of course, mice. The thing is, with the physical transformation, there is no emotional or internal transformation of any kind. When our narrator is changed into a mouse, he is still the same boy on the inside. He still thinks and talks like our narrator, and has the same personality. The same is true of Bruno Jenkins, who just cares about eating, whether he's a boy or a mouse. It seems like Roald Dahl is trying to tell us that it doesn't really matter what you look like on the outside, because it's what's on the inside that counts. It's cheesy, but true.
Our narrator's life is way better as a mouse. Think of all the fun he can have!
Roald Dahl should have turned the narrator back into a human at the end of the book. That would have been the right kind of happy ending.