Enough with the uncomfortable with house decor!

A lot of times, when my sims get that moodlet, it's because something is unfinished in the build, like missing wallpaper or flooring or something like that. Basic builds generally get the "decorated" moodlet if you put a few paintings or wall decorations up. More expensive items bump up the happiness. If you have a sim with the "snob" trait, they'll react more strongly to the less expensively decorated houses.