One common theme is growth. Just as a baby in diapers grows and develops, characters in these anime often experience personal growth. For example, they might learn new skills or overcome fears. Another theme is innocence. The characters usually have a certain naivete, similar to that of a young child. Friendship is also common, as the characters support each other through their growth processes.
In these types of anime, the theme of new beginnings is often present. Just as a baby in diapers is at the start of their life journey, the characters in the anime are often at the start of something new, like a new adventure or a new phase of life. The theme of kindness also abounds. Characters show kindness to one another, which is like the unconditional love a baby receives. And the theme of adaptation is there too. Characters have to adapt to new situations, much like a baby has to adapt to the world outside the diaper.
Well, in diaper anime stories, the theme of discovery is quite prevalent. Characters are constantly finding out new things about themselves, their world, or their abilities. It's like a baby in diapers exploring their surroundings. Also, family or community support is often a theme. The characters rely on those around them, much like a baby depends on its family. And of course, the theme of hope. No matter what difficulties they face, there is always an underlying sense of hope for a better future, similar to the hope that a baby represents for a family.