I do not agree. I find most characters in russian literature boring as hell and annoying, but they certainly have depth.
It's an introduction approach to game design. If the player can associate the cast with characters and not with objects - you have a much higher chance of him getting involved in the game. If the cast in the player head goes like "MILF1, Flat-chested-teen, idiot brother, some whore, teen-with-jugs, MILF 2 and 3...", there is a good chance he would not really care about the cast to get to know them. Let's take incest games for example.
If we want to keep up with the standard cast, there is a huge chance the player will just ignore them. Big sis is a bitch, lil sis is shy, mom is overworked, aunt is a perverted gold digger or just a slut (hopefully there are not twin nieces). It does nothing to the player. Seen it a million times. You can vary it a bit and shake it up, but if you want to use the classic trope - you must think of some way to make those characters interesting from the start. Maybe we get introduced to the bitch sister in a moment where she is vulnerable (maybe they just released some revenge porn with her). Maybe the little sister comes home so excited about her new book, that she ignores you. Maybe mom is not overworked and is actually an alcoholic trying to keep it together and we meet her drunk as fuck. Maybe we meet the aunt when her husband directly tells her he is going to his lover. Or that he is bringing a prostitute tonight for a threesome. Or something - yeah, she is a gold digger, but showing her humiliated or excited by such a thing will make her something more than "Milf2". Maybe the twins are dead (god, please!).
It doesn't need to be anything above a quirk, a trait, a habit. It just needs to make them interesting, captivating for just that moment. To not make the player think about them as blanks, but as characters. "This is my big sister and she is a bitch" does nothing for the player.
A good introduction by proxy also helps - maybe you meet the big sister... when somebody shows you the revenge porn! Great place to establish the MCs attitude, btw - will you get protective? Will you be happy she is humiliated? And now we have the seeds of a story arc for the big sister and at least one other character (whoever showed you that porn).
This is what I mean by interesting characters. Because we have a very short time to introduce somebody. And if the player gives no shits about them - there is a good chance they will just rush trough any depth shown later on. "It's MILF2, let me fuck her already!"