I would say, in the case of your example. It would depend on how it was handled. Just like anything else in a child's life. Whatever it is, can be carried to far, and made to be abusive.
If they were made to feel bad because of it, mocked, spanked, etc. that would be abusive.
If it's done to help the child feel better about waking up in a dry bed! Done with love and respect, and so they are aware it's not wrong, no not abuse.
There are lots of ways to handle it too, lots of good, or right ways. I think the abuse lays within the parent, or parents. The diapers, if use wrongly, would only be the instrument of abuse, not actually the abuse itself, there would be an underlining factor.
In such a case, it might be a good idea to offer the child a choice. We will have you wear diapers discreetly at bedtime, so you may wake up in a dry bed. Or, you will be responsible to take care of the wet clothes, bed sheets, blankets and any other wet items in the mornings when there is a wetting accident. Something along those lines will help to empower the child, and have them be more responsible for what they do and there own bodies.
There are many opinions on this however, some people think even that is to harsh.
Whatever the case, if it is truly done with love, respect, consideration, and the need to teach a child how to deal with a situation for themselves, it would be the right way to go.