Eh... I can see how you'd get the idea, but I feel it'd be at least ten times as confusing as mood avatars. I feel like it'd sorta decouple the layouts, which are pretty recognizable, from the users.

Sure, you could argue about only changing the color schemes and imagery. But that's more like a guideline, and there's plenty layouts that are edited from others. Liliana's Fluttershy layout, for example, is edited from mine. By me, no less. Imagine for the moment she'd suddenly switch to a really dark variant because of a... moody topic she's writing about. Now look at my layout.

So yeah. Sounds like a bad idea.