As a creator who's had a mature novel in Staff Picks (it's appeared on both desktop and app), and judging by what info I've gleaned from staff, mature novels can get featured if the M scenes are buried at least several episodes in and if the novel isn't just erotica and has a plot with some depth. It's still a bit fuzzy, since features are manually picked and are at the discretion of the individual staffer picking.
To note: for my own series, the first Mature episode doesn't appear until episode 13.