It depends.
Speed hacks don't give you an advantage, they only make your time more efficient. It's not like you need to finish the game before X time passes, so it doesn't matter after all. If I grind for one hour with a 2x speed hack, I will have used my free time most efficiently. This being said, I don't use speedhacks.
I have just started the game, and I already have cheated. My friend set the friendship of my Budew to the max via RPG maker. Is it cheating? Yes, of course, but I wanted to get roselia before I did anything else, even if that meant walking from one place to another, saving the game, leveling Budew and reseting if he wasn't evolving. I was going to get roselia anyways, but it saved me lots of time and tedious grinding
However adding 3rd party pokemons can lower the difficulty, or even nullify some encounters or events that were designed to make the game more challenging, possibly ruining the original experience, and maybe giving you a different one.
Also, what about making the game harder? Lowering your mons stats or giving them poor IV's in order to face a greater challenge is also considered cheating.
In the very end, you play games to have fun and as long as there is no PVP, if you feel like cheating your way through you should be able to do.