The line in question does not seem to indicate that Henry is being either particularly rude or malicious, rather that Catherine is perceptive of his finding amusment in folly to a degree that could lend itself to either encouraging or minimizing the destructiveness of said folly should he not check his behavior. Reading from "Mrs Tilney was polite enough..." onward to the end of the paragraph and his speach with Catherine displays his fault as being overly mirthful in the face of rather silly behavior rather than mean spirtedly contriving it.