Terraria doesn't have any kind of definitive style, other than pixels.
That's not entirely true, though. Leaving aside the art used in the Steam trading cards, which is at least
semi-official, but I talked about already, those pixels do create a distinct visual style that is recognizable. When people look through the mod or sprite art sections, it's pretty easy to tell the difference between a sprite that would fit right in as a part of the game, and those that just look out of place. Take, for instance, DakotaSpine's example.
There is a clear difference between:
This:
View attachment 9078
And this:
View attachment 9079
That is the nail on the head. While they are presented in sprite form, there is a recognizable quality to Terraria's assets beyond just having pixels. For one thing, they possess curvature and shading. Even in two dimensions, the form and shape of the pieces in three dimensions is at least implied. That is part of what looks so wrong about the toys. We may not have three dimensions in Terraria, but the way the items are shaped and coloured gives an idea of how it would look. So, when the toys aren't even close to those expected shapes, it's noticeable.
I'd even go so far as to say it's the key difference between the styles of Terraria and Minecraft. The latter is two dimensions transposed into three...that is, three dimensional representations of pixelated 2D sprites. The former, on the other hand, is clearly meant to be three dimensional objects represented in two. Even with just sprites, Terraria is able to convey depth and detail. Faces have features. Legs end in feet. Subtle use of light and dark tones convey lit and unlit faces on an object, as though it were in three dimensions and part of it in shadow. That's why the toys seem so glaringly wrong. Even though they're in three dimensions, they feel more flat than Terraria's sprites. When the look of them isn't just completely wrong (like the disastrous Night's Edge attempt) they strip away all that detail, presenting something that is oversimplified, and just seems to lack the "soul" of the game's designs.