There is a hint of humorousness in the last sentenceof "The Open Window" that relieves the tension in the preceding paragraph in which the reader learns that Vera is an unscrupulous and imaginative story-teller. If the story had ended at the preceding paragraph,
[Nuttel] had to spend the night in a newly dug grave with the creatures snarling and grinning and foaming just above him. Enough to make anyone lose their nerve.
there would have been a chill, unnerving element to the revelation that Vera is telling stories! After all, Nuttel needs a nerve cure and he literally ran from the house; this is maliciousness. But the last gently humorous sentence changes the mood through the tone of the narrator's voice and lets us know that Vera's focus is on the "romance" (which meant "adventure" in the 19th century) and not on malicious harm to others.
The last sentence does certainly give an ironic twist to our new understanding of what we have heard and seen in the story when we understand how blithely Vera drops into the story-teller's mood and persona.
I think it is definitely both - the last, stark paragraph represents the power of stories in the hands of a great storyteller, which Vera has definitely shown herself to be. Note the way in which, having tricked Mr. Nuttel into believing her ghost story with humorous consequences she proceeds to do exactly the same with her relatives - she is able to fool everyone with her storytelling abilities.
In Saki's story The Open Window we meet the niece of Mrs. Sappleton. She is about 15 years old and as we learn by the end of the story a competent liar. Her remarks to Mr. Nuttle are a creation of her imagination used to set him up in the end. Saki also sets up the reader in this same humor. After he rushes out of the house in pure terror because of her story, she immediately makes up another story to complete her ruse. Saki's remark at the end, "Romance at short notice was her speciality." is humorous and somewhat an indication of verbal irony, and "Saki uses a symbol ironically by having the open window, an object one might expect would imply honesty, as a symbol of deceit." However, "Saki's wit is at the height of its power in this story of a spontaneous practical joke played upon a visiting stranger. The practical joke recurs in many of Saki's stories, but "The Open Window'' is perhaps his most successful and best known example of the type."