Why is Corki bad?

no he had his spotlight for a couple of years

He's not great for this metagame. He was dominant for awhile because the top and jungle metas were very squishy; carries like Fiora and Jax dominated top, and Kindred and Graves were the kings of the jungle. In that meta, where few people are building huge amounts of both armor and MR, his mixed damage was an asset (since most people were only building one defensive item), and his strong poke was very useful.

Recently, the meta has shifted to more tanks in top and jungle (and even mid); Vladimir, Swain, Ekko, Gnar, Rek'Sai, Gragas, and so on. In heavy tank metas, his mixed scaling and reliance on poke become a liability, since he only builds one pen item. Other ADCs who are better able to handle or synergize with these tanks (Caitlyn, Lucian, Sivir) rose to the top of the heap.

His mixed damage was just too good. It was extremely difficult, if not impossible to itemize against him as a tank.

he needs something akin to a rework, meaning that some of his skills aren't really working in his favor.

He became almost a mage because they made his kit mostly magic damage. During early game, since your autos do half physical and half magical, the base 30 Magic resist on every champion diminishes your damage quite a bit, not as much as armor would since armor numbers are lower and it works somehow differently. He doesn't scales with AS as he used to either, since his autos don't have true damage you rather simply hit harder instead of faster.

Now he does scale with AD but he needs magic pen. and since there's no item with AD + magic pen you gotta make do, go for sorc boots and hybrid or straight up magic pen runes. He does have gatling gun in order to bypass some of the magic resist problem though, during 6 seconds only, so his damage is a bit more time condensed than that of the other ADCs damage (except for Caitlyn I guess).

Most of his damage is condensed into his special delivery jump too, it has a total of 750% AD scaling but you gotta use it correctly, if you're not cutting someone's escape route you loose a LOT of damage even though you keep the skill for only 1 minute every 8 minutes.

I haven't done the math but I think he's more powerful with AP items like:

which again, is sad considering he was the poster child for adcs throughout 2 or more seasons.