There are a lot of moves to choose from in Pokémon, but there is only one that no trainer should ever use unless they are purely evil. There are a lot of powerful moves in Pokémon, even some that damage the user in return. However, there is only one that specifically requires harm to the Pokémon's well being.
Ever since the beginning of the game, there have been some moves with negative effects for the Pokémon that use them. Hyper Beam requires the user to skip a turn afterwards. Moves like Double Edge deal some damage to the user in return, with Head Smash causing them to take half of the damage they just inflicted. Self Destruct and Explosion take that idea to the extreme by causing the user to instantly faint. Even with all of these negative effects, most of these moves are not among Pokémon's absolute worst moves available. There is one move that stands out for its drawback, though. In order for the player to get any value out of Frustration, their Pokémon needs to hate them.
Friendship is an important mechanic for a lot of Pokémon. There are several that can only evolve if their friendship is at the maximum level. There is also the move Return, which becomes stronger the higher the Pokémon's friendship is with the player, with its base power over 100 at maximum friendship, making it a decently useful move. On the other hand, Frustration operates the exact opposite way, with its power increasing the lower the Pokémon's friendship is. It's not only the only move in the series that works this way, but the only mechanic that relies on low happiness, period. Considering how easy it is to raise the Friendship stat in Pokémon, it seems like this was probably intentional.
Frustration Is Only Effective For Unhappy Pokémon
Even beyond the obvious problems with the move, getting maximum value out of Frustration is counter-intuitive and wasteful. Friendship is easy to raise in Pokémon, but difficult to lower. Friendship is raised by battling, using items on them, and simply walking with them in the party. Conversely, friendship gets lowered by the Pokémon fainting or using bitter herbal medicine on them. So to have a Pokémon with the lowest friendship possible, the player would need to have them faint repeatedly while also buying and using disliked items on them. Frustration is nowhere near worth that level of investment, especially for Pokémon that evolve via friendship. Actively decreasing a Pokémon's friendship may be the worst use of one's time in the series.
It seems strange that a move like Frustration would even be available to players. If anything, one would think that it would be a move usable exclusively by characters like Ghetsis or Team Rocket executives, purely villainous figures with no care for their Pokémon. Players have no reason to lower their Pokémon's happiness, especially not to the rock-bottom level required for a maximum power Frustration. It actively goes against one of the series' themes of trust and friendship between Pokémon and humans. Competitive and casual players alike have no use for Frustration. In fact, with recent evil Pokémon teams not being very evil, there's even less justification for Frustration's existence.
Frustration is a move that, by design, is useful to virtually no players. Raising friendship for Pokémon is not only easy, but universally positive, and Frustration can only see value from the player deliberately mistreating their Pokémon, which they have no reason to do. Only the most cruel, evil characters in the Pokémon universe could ever see any use from Frustration.