I'd say she's very niche. She doesn't counter much and gets countered by a lot, and every reason you play her for besides being a main, (waveclear mid, strong roams, mobility, burst) is done infinitely better by other champions; like ahri and kat which play the same role as AP carry assassins but do it better and have a fighting chance pre-6.
Diana has an identity issue nowadays, she's either overtuned to the point she's worth picking over the other champions, or she's just average and falls flat to being picked over ahri, ekko, kat, fizz, and LB. Even her matchups are way more frustrating for diana since she can't do much pre-6 while other mid laners can way easier, and even at 6 other mids still have a powerspike that counters hers.
IMO she needs a rek'sai/kindred level update. She has needed it for awhile, I think she was a decent pick in season 6 jungle when runic echoes was hype, but since then I haven't really seen a diana unless they were a diana main and even I don't find motivation to play her in most matchups dispite personally liking the champion. I see them do well, but experience > actual champion state.