'Facetious' means treating serious issues with deliberately inappropriate humor, or not meant to be taken seriously. It refers to a particular tone of levity or lack of seriousness that might be inappropriate in certain contexts. 'Flippantly humorous' best captures this sense of treating something with casual, perhaps inappropriate, humor. 'Amusing' is too neutral and positive, 'critical' is opposite in meaning, and 'casual' doesn't capture the specific quality of humorous disregard for seriousness that facetious implies.