You forgot to mention that this question is about "A fabula" by Mark Twain.
Answer:
to add ironic humor, as the character whom the others don't trust actually reveals the truth
Step-by-step explanation:
The fable talks about an aritsta who painted a picture and I leave it in front of a mirror, so that the image is softened and the picture is more beautiful. The cat, observing what the artist did, was amazed by the beauty and told the other animals how beautiful and elegant the artist's work was. However, some animals did not believe the cat's words and went to look at the artist's work for themselves. However, they positioned themselves between the artist's work and the mirror and were only able to see their own images reflected in the mirror, for this reason, they did not believe in the cat.
For this reason, Twain decides to bring the cat at the end of the story to reveal the moral of the narrative. With that Twain adds an ironic humor, because the character that others do not trust reveals the truth.