To clarify,
@Vingle, it's not the simple circles that makes them "dead eyes", nor the full-black eyes themselves. It's the overall design.
Like
@Toolbox pointed out, the Squishmallows and Bellizis are "blobs" with simple eyes tacked on, while things like things like Rilakuma have the proportions that make something actually cute.
As an example, compare the feature (eyes, forehead, mouth, head shape) manipulations in these images:
View attachment 2151159
To this:
View attachment 2151165
And then contrast with this:
View attachment 2151178
There's no clear body or head. It's just a blob. They're all just blobs with features and the occasional accessories sewn on to the base pillow:
View attachment 2151188
Even when we look at Korilakuma, and the body is a smaller, recolored version of the main bear, it makes sense because they're both bears based on traditional teddy bears. They don't then paint it yellow and say "Oh, this one is Kiiroitori now."