Rather than preservation of style, I'd just make 'style' (or perhaps quality) a quantitative magical properly. The higher the style of the desired result, the more difficult the transfiguration and the more skill needed. Partially flubbed spells cause the resulting object to be inferior on our magical 'style' scale.

As a magical properly rather than a mundane one we can side skip the problem where a mouse is far more complex and detailed than any clothing you could dream of.

The problem with the "Law of the Preservation of Style" is it failed completely when Conjuration concerned. And Conjuration (mostly) seems to run on the same rules as transfiguration, being a subdiscipline.