I also think it depends on how you style their house or how much thought you're going to put into it. If you're gonna go through the trouble (as I posted earlier), might as well make it worth it.
For example, the only villager in my town that has a fish is
Julian, and that's because I thought it would be really cute to put a seahorse in his house.

Then the rest of the furniture I gave him are very calm, white, and soothing, so his house, which started out as one of my favorites, is very peaceful. I feel like he's probably a masseuse or at least somehow very spiritual and he meditates a lot. lol, anyway basically I considered his personality.
So this is how I did a lot of their houses:
Marshal--I only swapped out a few of the dumber items for a couple of Nintendo items, just so his house would be a little bit of fun, but for the most part, I left it alone. I think his house fits his personality. And I really love to just hang out and watch TV with him. lol...well for like 20 seconds.
Pietro--I kept his house pretty much the same too, except I added even more balloon furniture. Which oddly enough is not his favorite thing. XD Too bad, because it fits his clown persona and is cute IMO. I happen to really like the balloon furniture, though it seems I'm forever alone on that one. ;_;
Sparro--He really had the ugliest house on earth originally. It took all that I could do, piece by piece to change it into something worthy of him. I think I did a good job, as now I like it in there. I tried to make it really brown with earth tones, and then added some Nintendo glowy items and the Palm Tree Lamp, which fits the color theme, IDK I think it all looks really nice now.
Stitches--Is my little boy, I love him and I wanted him to have a cute little room. Parts of it are super childish though, so it's a mishmash really of anywhere from toddler to pre-teen...lol, IDK...it's what I imagined would make him happy.
Muffy--I decided she had a chocolate fetish, so I kept her rococo furniture and just put chocolate items around it, like chocolate cake, a chocolate fountain, box of chocolates, cacao tree...and she just loves to sit down when I'm over, so it always looks like she's binged on it all.

<3 My kind of girl.
Merengue--Her theme is pastries and yumminess, so she got a birthday cake, a chocolate cake, and a wedding cake. A sweets player, and a couple of other delish looking items, her house looks like heaven.

I can't go in there a lot, it makes me hungry.
Moniqueand
Fuchsia--gave me a really hard time with their houses! Partly I think because they didn't start out too nice. So it was a lot of work, and...then some more work. I'm pretty happy now as I think they both look fairly good. But they aren't my absolute favorite houses or anything. Just as good as I could get them.
Carmen--I feel like she's a cute but cool little girl, kind of like Suzie from Calvin and Hobbes. Except super peppy and happy. So Suzie on drugs. XD I admit Carmen's house is my least favorite, even though she is actually my absolute favorite villager. But the real thing I'm proud of is that I got her to change the huge ugly robo table in the middle of her house. That was the Herculean task with her house. after that, everything else is icing on the cake.