I follow the same sort of a method (perhaps not quite as well defined as yours). In general, if it is hyper-serious or really ironically serious it gets a high rating. I consider the mechanic and any story/lore first. I also consider art/music/other design - if the art and music feel super serious I often score it high, but also if they seem really ironically linked (i.e. super cheery music in an ominous environment, etc.) I give it a high rating.
In general I usually end up rating most things around either 2 or 4. But yeah, ultimately my seriousness rating is heavily 'vibes based' on how much thought went into trying to make it serious. Stuff like a carefully timed goofy animation, taking every action really seriously (e.g. making it seem like jumping is a life or death decision) or a hilarious story generally convince me more than just slapping everything in a suit and tie or trying to add jokes in without linking them to the gameplay.