Mega Evolutions are meant to boost Pokemon by letting them evolve even further during battle. However, not all mons that utilize this mechanic have been able to do so successfully in the competitive meta. The reasons can vary from the original form being too flawed, gaining a situational ability, or not having access to useful moves.
We will look at the Mega Evolutions that were unable to do well in the competitive scene.
5 of the worst Mega Evolutions in competitive Pokemon
1) Mega Beedrill

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Mega Beedrill is actually semi-decent in Singles. It has 150 Attack and 145 Speed, letting it hit hard and fast. Unfortunately, it remains extremely frail and is likely to be knocked out easily by even neutral attacks, let alone supereffective hits.
In Singles, the presence of entry hazards like Spikes and Stealth Rock can wear down very fast. As for VGC, it is at risk of being double-targeted. Also, in both formats, its Bug/Poison STABs are not threatening many targets with massive damage. Even with its new ability, Adaptability, boosting STAB moves.
Also read: 5 best Mega Evolutions in competitive Pokemon
2) Mega Banette

Mega Banette's main problem is that it's slow; 75 base Speed is not bad, but it's quite lackluster for the team's designated Mega Evolution slot. It has an amazing 165 Attack but is most likely to be knocked out before getting to use it. It's even worse in VGC, where this doll can get hit twice, making its chances of affecting the game even worse.
The only use Mega Banette has is the move Destiny Bond in battle, which takes out an opponent who knocks out Mega Banette. And despite its poor Speed, thanks to its ability Prankster, it is guaranteed to pull off the move. However, it is a poor use of a Mega Evolution in either format.
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3) Mega Audino

Mega Audino is arguably worse in some ways than its base form. After Mega Evolving, Audino loses access to the amazing ability Regenerator, which heals it by a third of its health if it switches out. Instead, it has the ability Healer, which has a chance to heal the status of its allies. From that description, it's clear that the ability is useless in Singles and inconsistent in VGC.
The one thing that Mega Audino has is high defences. But the problem is, it isn't very helpful on the field as it struggles to deal meaningful damage with a Special Attack stat of 80. This leaves it as a passive creature that the opponent can ignore.
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4) Mega Ampharos

Mega Ampharos' main issue stems from its base form. After Mega Evolution, Ampharos gains a partial Dragon typing. However, as it isn't a Dragon-type in its base form, it does not have access to Draco Meteor, the most powerful Dragon-type move in the game.
Additionally, it is very slow with a base Speed stat of 45 and can only work well in Trick Room teams. However, Trick Room is not a common tactic in Singles, and Ampharos has neither the support movepool nor the damage output to justify using it in VGC.
Also read: How important are Pokemon abilities in competitive games?
5) Mega Glalie

Mega Glalie has good offensive typing and stats (base 100 in both Attack and Special Attack) and decent base 100 Speed. However, it's a pure Ice-type, which leaves it with several common weaknesses like Fighting and Fire. Additionally, its defenses are still the same as its base form (80/80/80), so it is unlikely to take more hits than it did before Mega Evolving.
The one main use Mega Glalie sees is as a lead in Singles. It sets up Spikes and then uses Explosion. The latter becomes an Ice move thanks to the new ability it gets after Mega Evolution, Refrigerate. But again, it's a poor use of a Mega Evolution slot on a team.
Also read: 5 best strategies you should use to climb Pokemon Scarlet and Violet competitive ladder in Singles