For me, one of the biggest factors of beauty is consistency – if I’m building a farming village then all houses should work well as part of a cohesive whole, if I’m building a particular house then every decoration in it should be telling the same story rather than, say, one room being bright and modern while another has an antique and cosy feel.
Unfortunately, in games this usually turns into an expression of “variety=clutter”, and that’s restrictive. Of course, the opposite idea “variety of decorations = high score” can be just as painful.
One game which handles the idea very well though, IMO, is Terraria. It gives you a huge palette of decorations and design options – dozens of furniture sets, dozens of paints (and of course you can paint furniture and such, not just walls), printed and thematic wallpapers, statues, lighting options, etc. – but doesn’t restrict you to using a particular set together for a bonus. To make a valid house for an NPC, you have a minimum and maximum size constraint and a certain furniture set (a “chair” which can be anything from a stool to a couch to a bathtub, a “table” which can be a workbench or a dresser or so on, and a light source), the style of those sets of furniture have a purely cosmetic impact. It is, however, easy to theme a room even when you’re mixing and matching styles; e.g. you can get a really cool spooky/horror style from using just the Halloween furniture but you can also mix in bone, crystal, corrupt, demonic, boreal/arctic and even hallowed materials to complement each other. Juxtaposing some hallowed bricks and pearlwood with the leering skulls of demonic furniture, for example, can create a focal point which the player considers beautiful, but from the game’s perspective the two different sets are doing the same job.
I think that one option for Stonehearth is to draw from Terraria’s “home” system – come up with a set of furniture that counts as a minimum set for a home (that might be just a bed), then have a “comfy” home set and higher levels going up to “luxurious” or even “regal” if you want to get into having royals/leaders/aristocrats. So a home with a comfy bed, a chair and a desk might be considered comfy, whereas a grand palace which only has statues might not actually be so comfy as that rustic house.
I like the idea of different hearthlings having different ideas/expectations of beauty, though. It could be tied partly to their traits – as an example, hearthlings with high Spirit like rustic, comfy, home-style furnishings, while hearthlings with high Mind like artistic flourishes and good lighting, and hearthlings with high Body like statues, trophies, fences, and “solid” decorations and structures.
There’s also the question of nature vs artifice – some hearthlings will prefer trees and flowers, while others will prefer columns, paintings, stained-glass windows… I think the best way to handle that is to not have any decorations ever have a negative impact on beauty, even if a hearthling isn’t into that kind of decoration; only have “problems” and ugly things (e.g. logs needing to be hauled, or rotten food, or a warning totem built by goblins) have a negative impact. So if a hearthling isn’t a fan of trees they won’t get upset about being surrounded by them, they just won’t appreciate that natural beauty like another hearthling might. However, if you add a few choice items, you can preserve the natural beauty and complement it (perhaps with a statue and a park bench) so you give that hearthling something to enjoy without changing the overall aesthetic.