Abilities are there to make Pokemon better than they already are. In most cases, they give Pokemon a little push to great viability. In some, they're signature abilities certain Pokemon hang their hat on, to the point that they would be completely unviable without it. That's the case for signature abilities such as Mimikyu's Disguise and Ditto's Imposter.
Unfortunately, not every single ability is great, and that includes signature abilities. In fact, some of them, while essential to the Pokemon's lore, hold the Pokemon back from being competitively viable. Some of them are even detrimental, worsening their battling abilities rather than helping them. While most abilities are at least helpful, these ones prove to be more of an inconvenience than a lifeline.
8 Morpeko - Hunger Switch
Generation VIII produced what should've been another run-of-the-mill Pikachu clone in Morpeko, but it fortunately came with a nice twist. Its Hunger Switch ability causes it to switch between Full Belly and Hangry Mode every turn, with its signature move Aura Wheel subsequently switching between being an Electric-type and Dark-type move, respectively.
However, while it's a welcome change of pace in terms of character design, having to deal with a move switching types every turn is pretty taxing. It's very likely that you'll end up in situations where you need the Dark-type variant but only have the Electric-type variant, or vice versa.
7 Galarian Slowking - Curious Medicine
Believe it or not, Galarian Slowking has been very viable, reaching Smogon's OverUsed tier of Singles battling. However, none of that credit goes to the new ability, Curious Medicine. To be fair, the ability is only meant to work in double battles, but it doesn't provide many useful benefits there, either.
Curious Medicine removes all stat modifiers on the allies' side, and not the opponents'. While that can be useful for removing negative stat modifiers, it's better to just switch out the Pokemon with stat drops. Furthermore, getting a stray stat boost knowing that it might get hazed out doesn't sit right with most players. As a result, most competitive sets run the much superior Regenerator instead, for both Singles and Doubles metas.
6 Kecleon - Color Change
It's understandable why Kecleon has the ability it does, considering its chameleon-like features of going invisible don't really make a difference in the interface of a Pokemon battle. Unfortunately, the signature ability named "Color Change" isn't really viable at all.
Color Change changes Kecleon's type based on the attack it takes; taking on a Thunder will make it an Electric-type, and so on. While it's a cool gimmick, it's a terrible foundation for a competitive strategy. You're left at the mercy of your opponent's moves every single time, and that gives them freedom to manipulate Kecleon's type to their choosing. It, fortunately, has Protean as a hidden ability, which makes Kecleon take on the types of your attacks, not your opponents'.
5 Yamper - Ball Fetch
Yamper's signature ability, Ball Fetch, is one of those abilities that falls under the category similar to Run Away and Pickup, in that, while it's not detrimental to a Pokemon's battling prowess, it's totally useless from a competitive sense.
Ball Fetch allows this Electric-type to keep the Poke Ball from a failed catch, if it doesn't have a held item. While that can be marginally useful and save you some extra cash, it only happens on the first throw. Additionally, Poke Balls are not really that expensive (nor are their upgraded versions). The silver lining is that Ball Fetch turns into Strong Jaw, a very good ability, when Yamper evolves into Boltund.
4 Sableye - Stall
This ability is the first on this list to actually be detrimental to the Pokemon. While Sableye has a few better abilities such as Keen Eye and Prankster, you're cursed with one that has Stall as an ability every once in a while.
Unless it's using a priority move like Quick Attack, Pokemon with Stall are essentially guaranteed to move last. While that could be nice for moves that are stronger when executed last in a turn, Sableye only has one of those: Payback, and it's not like it has the Attack stat to really make it useful. Granted, it's not like it's putting a good Speed stat to waste either (50 Speed), but losing the speed battle against even the slowest Pokemon in the game doesn't help.
3 Delcatty - Normalize
To prove that Skitty and Delcatty are some of the worst Pokemon in battle, here's an ability that makes them even worse than they already are. Normalize, for all its attack-boosting and synergy with their STAB typing, is a terrible ability.
While the Normal-type is a solid type to carry defensively, it's one of the worst ones offensively for moves, since it can't hit anything for super-effective damage. While Normal-types usually compensate for that by having access to tons of coverage moves, most Delcatty are stuck with dealing Normal-type damage for every attack they do. Normalize is a devastatingly bad ability, especially for a Pokemon that's already subpar in battle to begin with.
2 Archeops - Defeatist
If abilities didn't exist, Archeops would be an incredible Pokemon to use in battle. It has an excellent 567 base stat total, with most of that going to its sky-high Attack stat of 140. Unfortunately, that ability is precisely why Archeops is barely competitively viable.
The Generation V fossil is the only Pokemon with Defeatist, an ability that halves the Pokemon's Attack and Special Attack if its HP goes below half. While it was perhaps made to nerf an absurdly strong Pokemon, it's terrible for its competitive viability. Keeping that HP above half would make it an excellent attacker, but that's a tough task considering its mediocre bulk (75 HP) and terrible defenses (65 in both Defense and Special Defense).
1 Regigigas - Slow Start
Regigigas is an eerily similar case to Archeops. Among the big three of Pokemon that are unviable because of detrimental abilities, Regigigas probably has it the worst when compared to Archeops' Defeatist and Slaking's Truant (which isn't on this list because Durant has it as a hidden ability).
Regigigas should be an incredibly strong Pokemon to use, having an excellent movepool and Legendary-level stats appropriate for its Legendary status. However, it's completely unviable thanks to Slow Start, an ability that halves its Attack and Speed for the first five turns it has been brought out. That is insanely detrimental, as Regigigas essentially becomes death fodder for five turns. Staying alive for five turns without your offensive capabilities is even harder than keeping your HP above half in Archeops' case.