Cheating in a single player game is like looking up a spoiler for the end of a movie. There's nothing wrong with it, but you should be aware the game may not be as much fun afterwards.
Personally, I'm the type who will play a game straight through for the story and experience of it, then go back and do cheats to expand my enjoyment of the journey.
Again, this is all talking about single player games; cheating in multiplayer games is inexcusable as it will always negatively impact other players and their enjoyment of the game.
To use a real example, Celeste is a very difficult single player game, and so the developers actually built in a cheat mode that any player can enable. I'm not going to lie, there are times I was tempted to turn it on.
But, personally, I'm really glad I held myself back! If I had turned on Celeste's assist mode, it would have cheapened the entire experience for me.
And to mention the other side again: Celeste's developers purposefully added Assist Mode for a reason, so anyone who truly needs it should go ahead and use it. Just, make a real effort to persevere if you can—you just might find yourself doing something that seemed impossible at first.
In every open world RPG I will always give myself unlimited inventory and durability, I can't stand those mechanics.
You're not being dishonest or unfair when you mod or (use a) cheat in a single player game, you're free to do as you please, however if other players are involved, there are categories of mods that would fit that description.
