According to a Bulbapedia page:
"All wallpapers and interior items can be grouped into several sets based on common criteria for obtaining them. Each set also fits a general theme and naming scheme described below as "category". Sets D and E actually consist of several categories each, corresponding to types. These type categories are the sets that a group of visitors may draw a gift from if they all share a type.
The ways to receive a decoration are:
- Unlock a new difficulty for a minigame.
- Earn a 5-star rating on a minigame.
- Raise the active Pokémon's affection to 25, 50, 75, 100, 150, or 255 points.
- A bug can also grant a gift from the active Pokémon when a different Pokémon's affection is raised to a sufficient level when staying at Hotel Richissime.
Receive a gift from visitors.
- Visitor gifts draw from different sets (from A to F) depending on the visitors' combined affection levels, but occasional inconsistencies make describing the bounds of affection required for one set or another difficult. Also, minigame gifts do not stack. That is, if the player unlocks two difficulties in a row or gets 5 stars on any two games or gets 5 stars on a game and that unlocks the next difficulty, when they exit the play mode, they will still only get the latest gift (with unlocking occurring after the player earns their rating)."
Apparently I woke up too early to figure out exactly what this means you should do. Try getting five stars in hard-difficulty minigames, getting online if you can (and maybe setting out a Poke Puff) so that visitors can visit, stuff like that.
Here's the link to the page:
https://bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon-Amie_decorations
Wish I could be more clear, but hopefully this helped somewhat.
