See the ironic thing here is that (SPOILER)the Sans boss fight is not annoying or unfun at all, in fact it's the one part of the game I'm considering having a save game right before so I can redo it whenever I want. Because it's damn fun to fight him.
And, well, (SPOILER)I don't care much if it's intended or not; I consider it bad design either way. The only justification I can imagine for intentionally boring the player is if there's a payoff for it that absolutely requires the player to have been bored for it. Genocide Route would have worked equally well (for me at least) if you cut out the boredom of grinding. Hell it doesn't even touch upon the whole thing being a time-intensive grind.