Mr. Slushy Dawg asks:

“Pickup is not useful. That is all.”
Along the same lines, what are other universally useless abilities?

Well, there’s not a lot that are literally useless – even Pickup occasionally does something if you’re fighting an opponent who uses berries – but there are a couple with no in-battle effect whatsoever.  Honey Gather is used only to generate Honey, Illuminate only increases the wild Pokémon encounter rate, and Run Away only allows you to escape wild Pokémon without fail.  An honourable mention should go here to Zygarde’s Aura Break, which improves his matchups against exactly two Pokémon – Xerneas and Yveltal – but otherwise does absolutely nothing.

Colour Change and Zen Mode are a special kind of bad, because they actually change you in ways that may mess up what you’re trying to do (denying you your same-type attack bonus and potentially letting your opponents hit you for super-effective damage in the case of Colour Change, or completely rewiring your stats in the case of Zen Mode).

Ones that are merely very bad include:

Big Pecks, Inner Focus and Keen Eye – your mileage may vary, and these are less useless in single-player, but the immunities they grant (defence drops, flinching and accuracy drops) are very rare in competitive play, and with the exception of flinching they can be negated by switching out.

Wimp Out/Emergency Exit – in theory interesting, especially with Golisopod’s First Impression signature move, but in practice this ability is usually just an inconvenience.

Heavy Metal/Light Metal – these abilities are each good for exactly one thing: Heavy Metal increases the damage you deal with Heavy Slam, and Light Metal reduces the damage you take from Low Kick and Grass Knot.  What’s more, they’re only available as hidden abilities, all the Pokémon that get them have much better abilities to choose from, all the Pokémon with Light Metal resist Grass Knot anyway, and as the crowning insult to injury, Metagross is so heavy that even with Light Metal he’s still in the highest damage class for Low Kick and Grass Knot.

Unnerve – your opponent can’t use berries.  Sure, some Pokémon can be played as Salac Berry sweepers, but even when they show up, you still need to have your Unnerve Pokémon in play at exactly the right moment to mess with them.

Stall – Sableye’s signature ability, which allows you to… always go last.  No, I don’t know why either.

And of course there’s Truant, Slow Start and Defeatist, but it hardly seems right to count those, since they’re deliberate penalties to Pokémon who would otherwise be extremely powerful.