These stories are popular because A) things like divorce, mental disorders actually happen and most mainstream frum media never make any mention of it. This makes it “new and exciting” to readers. B) Stories that are realistic strike a chord in the reader that allows them to connect to the story and the characters. Can you really get into a nuclear physicist turned Ba’al teshuva who is the only one around who is able to stop a large scale terrorist attack on the population all while battling shidduch issues.
As for should it be counted as frum reading material I don’t believe it’s healthy to read so many stories with a “happily ever after” ending. Throughout our history we have always faced tzaros and often didn’t have perfect ending, but what’s always kept us strong was that we believed in H”BH and made do as best as possible. If a story can impart that on a reader I can’t see why it wouldn’t be good reading material.