Quoting the interview with ZUN in BAiJR here: "For Phantasmagoria of Flower View, the theme I made was "enjoyable while playing and after playing". Usually, you may think "playing is fun", and it's exciting to do so, but if playing is all there is, then it's unexpectedly not fun." If this game is enjoyable while playing but not after, then maybe it has something to do with the overall atmosphere and story in addition to the negative character art. My guess is that those three factors will be overall depressing, as opposed to PoFV's "Feel-good quality".