If Pokémon Go were an offline, single-player game then yes it would be okay. But since Pokémon Go plays in a server with multiple other players then if you cheat, you are ruining the game for other players. Cheating offline in a single-player game is fine because you are changing the game for your own preference without ruining the fun for others. If you cheat online then you might get banned. If you cheat in multiplayer then you are a jerk.
It isn't fair to cheat in Pokemon Go because it makes the game difficult for other players, particularly those who are just starting out. Also, cheating takes away the thrill of working hard and earning through play, which makes the game less exciting and will result in fewer people playing because it's just too easy to win.
While slightly less substantiated, some users have also suggested that “Pokémon Go’s” ban algorithm is tied to suspicious activity reports from individual players. One such case that’s garnered lots of attention over the past few days happened when one user told his friends to report his legitimate account to deliberately test the system. Just hours after the false negative flags had been dispatched, the account was supposedly taken offline.
The whole point of Pokemon Go is to get out and exercise. The fun is waiting for new characters and figures to appear so that a person can add to their collection. It is not fun to cheat the game because then a person will not know what they've really accomplished in the game.
Cheating in any game is never fair to the other people, that play it. It gives one person an advantage in the game that other players do not have. They have better chances at being successful in the game and make the game less fun for everybody else. In a game like this where you are battling your Pokemon against other players' Pokemons you have an unfair advantage. You are able to make your Pokemon better then others, which could make you almost invincible in the game.