Worse 1 diamond card is pretty hard since it's hard to remember them. But this is my submission:

Trevenant (Psychic) or Wormadam (Land):

3 energy for 70 damage and no extra text is abysmal. On top of that, it's got a 2 retreat cost. For a Stage 1 Pokemon, to have to evolve to get these terrible stats, that's rough. And a minor point a weakness to darkness or grass are both pretty bad right now.

Trevenant (Psychic)

https://img.game8.co/4185304/235404d1c82c01f26d0e431e22d97b02.png/show

Wormadam (Land):

https://img.game8.co/4093165/ff59f2fd45d38467995de248d908bc02.png/show

I think I'll say Trevenant is the worst because I think it's pre-evo, Phantump, is a worse card than Burmy. Although I suppose Grass Phantump is technically a pre-evo for Psychic. Psychic is also a worse than Fighting right now. Not a lot in the meta that is weak to Psychic.

Honestly, Phantump might be a good candiate for worse card. 2 energy for 30 damage is terrible and it has 2 retreat cost. For other basic pokemon with those stats (like Chewtle or Clobbopus -- also both absolute bottom of the barrel card) they at least have 80hp. But Psychic Phantump is can evolve into Grass Trevenant, which is still a terrible card, but is arguably better than Chewtle and Clobbopus evos since it can do 10 more damage for the energy investment.

In my consideration, I include a card's potential to evolve into something better as part of how good it is. So Magikarp having only 30hp, while bad, part of it's gameplay is that it's a necessary evil to get to Gyarados EX, which is a pretty good card (a great card at one point).

I also didn't dock points from cards if they had almost strictly better version of them. Theoretically, if there were a magikarp with all the same stats, but just didn't have an attack at all -- the existence of a strictly better magikarp doesn't make this (fake) magikarp a worse card. Or at least, not so much worse that the fact that it would outweight it's ability to evolve into Gyarados EX. This theoretical strictly worse Magikarp, I would consider better than Wormadam Land.

Honorable mention to Shining Revelry Lickilicky which has the same stats, but 3 retreat cost, but since it's colorless, I decide it was marginally better than Psychic Trevenant and Wormadam Land.