The ball of yarn on the bed is the Destiny Knot, whose in-game description reads: "If the holder becomes infatuated, the Pokémon that the holder is in love with becomes infatuated as well."

Is there a tag on danbooru that applies in cases like this?

Zurreak said in comment #2586136:

The ball of yarn on the bed is the Destiny Knot, whose in-game description reads: "If the holder becomes infatuated, the Pokémon that the holder is in love with becomes infatuated as well."

Is there a tag on danbooru that applies in cases like this?

We have destiny knot (pokemon), any other tag might be a bit too subjective, I think.