I kinda disagree with a lot of that.

The fact that the monsters try to kill you on sight despite being goofballs is exactly where the grey area lies. They need the soul of a human in order to escape the underground and return to the surface, but they're all kindhearted enough that the thought of killing a human disgusts them, so they're happy to find any excuse they can to spare your life. This is especially the case with Asgore, who would rather stay in the underground and do nothing than use one of the human souls he already has to cross the barrier and proactively hunt humans on the surface- he's too kind to want to kill humans, even if he knows he has to do so for the good of his people. That's why talking things out with the monsters is important.

I think making the genocide route painful from a gameplay experience is an effective way of making the player feel bad for their actions- because it shows that despite the game content being there, the game is actively trying to resist your efforts every step of the way. It wants you to stop and question why you're trying to complete game content for the sake of completing game content- and by persisting along the genocide route, you're justifying what the game is talking about when it openly villainises you. That, in turn, makes Undertale and the characters who inhabit the game world feel more real, as opposed to video game characters trying to relay an anti-violence message. It's not that they want you to learn a lesson, (besides Sans and Papyrus) it's that they want to protect their own lives. On a genocide route, most of the major characters couldn't care less about you, they only know that you need to be stopped by any means necessary because they recognise you as a legitimate threat. The fact that the game even went so far as to reduce encounter rates until all the monsters in an area are dead shows a ton of thought and attention to detail by taking that game mechanic to a logical extreme, and the characters react very quickly and realistically to your change in attitude. Nothing in the Genocide route suggests that a lack of thought was put into it. You may not like that it frustrates you, and you're not supposed to. If making the player hate the game saves their lives, then they'll happily go through any means necessary. As such the game doesn't need to directly moralise in order to get its point across, even if the genocide route is otherwise incredibly blunt. It's the kind of storytelling via game mechanics that was sorely needed in ICEY.

I think it's important that frustration can be seen as something that can be a useful addition to the artistic value of a game. However, a lot of gamers judge games by conventional game design wisdom and their own raw first impressions. Undertale breaks some conventional wisdom when it comes to game design, but if you assume that everything is intentional while you play through it, then it really starts to click.

With ICEY, it's harder for me to say that the game's shortcomings were meant to add to the artistic value of the experience. My dislike of the narrator can be explained by them intentionally wanting to make him come across as dorky, but it's harder for me to rationalise the copycat meta elements and the obtuse hidden story as being anything other than being earnest failures of the game.