Pokémon Sword & Shield are letting players team up with other trainers to take on giant Pokémon in battle, but the games have a problem in the form of NPCs using a Magikarp in combat and causing the entire group to lose. One of the biggest new additions to Pokémon Sword & Shield is the Wild Area, where it's possible to fight powerful Pokémon in their Dynamax or Gigantamax forms and potentially catch them once they are defeated.
If the player sees a pillar of light in the Wild Area, they can approach it in order to start a Max Raid Battle. It's possible for up to four players to take on a Max Raid Battle at once, in which they will face a Dynamax/Gigantamax Pokémon. The enemy Pokémon is usually incredibly powerful and there are other conditions that will cause the battle to end early, such as four allied Pokémon fainting in battle or ten turns passing. If the party manages to defeat the enemy Pokémon, then they will all have the chance to catch it. The Max Raid Battle Pokémon will change over time, meaning players only have a limited opportunity to catch them.
If the player takes on a Max Raid Battle on their own, then Pokémon Sword & Shield will provide NPC trainers to help them. It's these NPCs that are causing issues, due to how some of them bring a useless Magikarp into battle. According to Polygon, there have been complaints on social media regarding the Magikarp users, as they have the potential to cause players to lose Max Raid Battles, either by wasting turns, fainting too easily, or just not contributing to the battle.
The ability to play Max Raid Battles over the Internet means that players can also grief their fellow trainers by bringing Magikarp into the fight, simply to have it use Splash and waste turns. There have been people who have defeated the Elite Four with Magikarp in the past, but these were level 99 Magikarp that were trained specifically for the purposes of battle, while the ones being used in Max Raid Battles exist only to infuriate other players online.
Over half of the Pokédex was cut in Pokémon Sword & Shield, yet Magikarp still made it in. Pokémon Sword & Shield had a number of controversies leading up to launch and the removal of Magikarp would have been a big deal if it had leaked before release, as people still love the useless fish Pokémon despite its flaws. It's easy to see why the developers would have thought that NPCs with Magikarps would have been a funny sight in battle, but it's also costing players victory during some of the most difficult battles in the game.
Source: Polygon