It defeats its own purpose.
It is supposed to shock player with drastic and unexpected change of tone, but its only defining feature that made it somewhat known is said change of tone in the first place, making it no longer unexpected.
Same with undertale really - it's only known for its meta narrative, which supposed to be surprise but since it sells the game there is no surprise.
My point is if the game can't stand on its own merits, such as gameplay or production values, then its turns and twists amount to nothing, there is no value in them.
Compare to Cave Story that has cutesy appearance but also top notch gameplay and fine tuned difficulty curve, making it cult classic hailed as one of the best platformers, not as one of those pretentious "it was grimdark all along" pieces of shit like eversion or undertale.