Sometimes, all a Pokémon needs in order to do well in battle is a good ability.
Take, for example, Blaziken, who spent the first two generations of its life as a serviceable Fire/Fighting-type, but then upgraded itself to a deadly sweeping machine in Generation V, once it got access to Speed Boost.
The very opposite fate befell poor Gengar, who lost the ability Levitate and had it replaced with Cursed Body, as of Generation VII. Thankfully, though, its Mega Evolution has Shadow Tag, which is leagues better.
Point is, an ability can either buff a Pokémon or make it infinitely worse. This is precisely why some Pokémon have unique, gimmicky abilities - to balance out their stats. For Pokémon like Regigigas with Slow Start, this is debilitating, but for Pokémon like Shedinja with Wonder Guard, it offers a little more hope.
But those aren't the abilities we'll be talking about.
No, too many "worst Pokémon abilities" lists talk extensively about signature abilities, such as the Skitty line's Normalize and the Archen line's Defeatist. This list, however, is going to be about those other bad abilities; the ones that aren't exclusive to one Pokémon, or one evolutionary line.
Hell, some of these abilities are actually pretty common and widespread.