How does the image of the kettle at the end of the first stanza imply that the butler has treated the kettle poorly?

You can say that the cracks(if there are any) show that the butler has not cared for it, if that's the question, you haven't provided a picture.
You can talk about the physical damage proves that the butler was careless about it and did not treat it gently.

as extra, you can say that the kettle is a metaphor for someone else the butler had treated poorly, the physical scars on the kettle similar to that person's emotional scars.