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See, I don't have a problem with that design. It looks like a cute little doll, like those nice little concept drawings from around the time it was announced. But the design being used in-game is different, and it looks awful.
When looking at some .GIF of Sackboy making yet another "hilarious" angry expression, I couldn't quite place my finger on what it was that seemed so wrong. Then I saw a "real" Sackboy made out of cloth, and it was obvious.
The Sackboy shown in the concept art and Sackboy renders, and the Sackboy doll shown in Guybrush's post, are dolls. They have big, very simple, and soft mouths that are loosely sewn together, little black plastic eyes pasted on, and plain unmoving features. They look like something made of soft stuffing inside a limp piece of cloth, only come to life and imbued with the power to move its limbs, neck, mouth, etc. And in plenty of the screenshots, you can see a default Sackboy model looking like that, a cute little sewn doll with a simple smile.
Yet there are Sackboy models in-game with moving cheek muscles, lips that move independently of the mouth, rigid teeth, and deeply-set eyes. I understand things like a little furrowed brow, or a big smile that wrinkles the burlap; those give the Sackboys some emotion while still being plain. But they're giving these things human-esque facial musculature. When the cheek muscles rise up, the brows go down, and the eyelids start blinking (especially when they've got those painted white eyes that conform to the eyelids and cheeks), Sackboy's eye resembles an organic eyeball in a socket, rather than a plastic doll eye glued to a stuffed cloth sack. And that's in conjunction with those lips sliding over these hard-looking teeth (which only seem to appear in certain facial expressions) as the mouth moves around.
It looks less like a doll come to life, and more like some alien organism that happens to have doll-like features.
In-game those features might not be very noticeable, but thanks to the overproliferation of people showing off those "cute" faces, it's all I can picture when "Sackboy" or "LittleBigPlanet" is mentioned.