I'll argue that Shedinja might be a bad and gimmicky Pokémon, but at least it does something, and is actually quite something out of the ordinary. It stands out. It has a unique ability. Concept-wise, it's pretty original. A magical shell shed after Nincada evolved, impervious to most attacking types but will crumble like tin foil when hit by something its magic can't stop. This might not make it a good Pokémon, but I certainly don't think it's the worst ever. It stands out, which is more than can be said for, say, Dewgong, Seaking or Maractus.
Then again, I suppose it all comes down to how you define a bad Pokémon, or the worst ever. In my opinion, this largely means it has no discernible use in battle, a half-assed back story, an unremarkable design, does not feature prominently in any medium, and doesn't stand out among other Pokémon in any way. For me, Lumineon represents the pinnacle of all those. 'mons like Mr. Mime, Stunfisk, Shedinja and Luvdisc are at least remarkable enough that we keep bringing them up. They are so silly/bad/ugly that you have to remember them. Not so much for the likes of Noctowl, Phione, Seismitoad or Swalot. They're just there to fluff the Pokédex.