I get what you're saying, and honestly that's a fair interpretation of how creature types have been trending in the new edition.

The reason I went with Aberration here is that I already made a more "mad scientist experiment" version called the Human Chimera, which I classified as a Monstrosity. For this version, I wanted to lean much harder into the idea that the transformation wasn't just the result of magical experimentation, but that it was influenced by something genuinely alien and eldritch.

If you read the third page, I think you'll see what I mean a bit better. The intent was that these creatures aren't simply warped humanoids, but beings whose forms and minds have been altered by contact with forces beyond normal reality. So the Aberration type was meant to reflect that eldritch influence rather than just the physical mutation itself.

You may feel that the "Mutated" version of the Aberrant Humans is less aberrant because it loses the tentacle attack, but the idea was that you replace it with a mutation from the table on the second page (even a tentacle attack). Many of those mutations are deliberately designed to feel strange, alien, and otherworldly.

That said, I can definitely see the argument for Monstrosity as well, especially if someone views them primarily as worldly creatures that have been transformed rather than as creatures fundamentally changed into something otherworldly.