It is intentional that deco isn't shown on save thumbnails.

There are a few reasons for this. The original justification is to stop art saves from being shown on front page, in an attempt to have only "functional" saves there. Functional saves don't need deco, so will look good regardless, so the hope was that making art saves less appealing would make them get front page less. Obviously things have changed a lot in the years since the change was made. I don't mind art saves anymore, as long as they look good and aren't a block of DMND.

Even though we don't regularly remove saves from front page anymore, the decoration block on thumbnails discourages creating saves with just deco, you actually have to put in some effort to use TPT elements and make your saves look acceptable without deco.

These days, CGI is also easier than ever (due to the addition of Lua scripting), so blocking deco also helps reduce CGI usage, making it less simple / appealing. We don't want users importing images into TPT and getting front page based on that.