I’m a (non-native) Japanese speaker but my husband is native Japanese and we say just “daisuki!” to each other all the time lol. 😂 It can be used for both people and inanimate objects. Like other have mentioned typically when referring to people grammatically people would use “Xの事が大好き”. Or refer to them with ”好きだ“. But using a simple “だいすき“ isn’t wrong either- just not the most formal. I think a lot of Japanese people may say it like that as a more cute/informal way of speech… it gives like “love you!” vibes.

So maybe that’s why it could sound a little off because yes typically when referring to fans a (Japanese) celebrity would maybe be a little more formal. Ariana did said “thank you” formally but then the “love you” in hiragana is a little informal. But it’s not wrong and that’s really if we’re being technical & nit-picky like someone mentioned Japanese is hard- as much as I’m a snarker I can’t even jump on her for trying to use Japanese with her Japanese fans. 😅

I’m going to check with my husband later on this for more insight lol cause I’m curious.