I assume you're talking about
?
Diana's early game (pre-6) is really bad. She probably has the worst pre-6, perhaps tied with Kassadin and Kayle (though Kayle is mostly top now) since Akali's rework. Since damage it so high, it has become harder and harder to survive the early game. Her base AD is one of the lowest for melee champions (53-104), compared to other AP melees midlane like
(58-125.2),
(58-109),
(55-106),
(58-109). Her attack speed and attack speed growth are also the lowest amongst these champions. This makes it so difficult to CS with her early despite her clunky attack speed passive.
If you somehow get to midgame, you're usually outclassed by other champions who can do your job better as a fighter/diver such as


. Itemization for her is super limited. She doesn't get the luxuries of an all-you-can-eat buffet of stats like
. She can't get good healing items
or
(
AD is nearly wasted on her since she doesn't scale off AD and all her damage except R is AOE). Her best fighter build is
+
, but Rod of Ages takes ages to stack and delays her midgame, while Nashor's Tooth hasn't been a good melee item in forever (and Stinger was nerfed 1-2 years ago). So she's forced to build into an unhealthy AP-nuke by starting either
(which she only buys for the stats - she can barely make good use of the passive) or
(which is horribly slot inefficient). Plus, Diana has horribly synergy with runes - especially keystones. Her best rune is Electrocute but you usually can't use it until level 6. She can't use conqueror effectively until level 6 too, and you've usually used your entire rotation by the time you get 5 stacks. You can't use Aftershock until level 6 too (unless you're in a melee matchup).
And if you ever get to the late game, you've fallen off in relevancy most of the time.
I get that a lot of people dislike Diana because of her unhealthy burst combo, but that's literally the only thing she can do at this point, and not even that well anymore. They changed her passive nearly 2 years ago and then just gave up work on her it seems. She seems like she's not high priority on the mini-rework or balance changes list, but she's barely functional as it is. According to OP.GG, she frequents a 46-48% win rate most of the time, which is abysmal consider her low pickrate and how she is a relatively easy champion to play.