I've always believed that it's good practice to be praying to them once every night or two, so that they could mutate any one of my Pokémon someday and make it seem special, one-of-a-kind.
I prefer to just give mine nicknames...I could sacrifice a goat every full moon to the RNG and I think it'd probably still ignore me! xD
However Some Pokemon doesn't evolve by Leveling Up aka Pikachu .
They need something else to evolve like Stone , Trade , Item , Place , Time etc etc .
Also All Combee doesn't become Vespiquen .
Only A female Combee evolve into Vespiquen.
However , It also consider a Evolution.
Honestly , It seem GF Simply using Mutation to drew eye From Evolution. Because Diancle does seem like a Evolution that Some Carbink might go though .
You're overlooking the fact that you can force all of those to occur, and it will happen to every single Pokemon that fits the criteria. If you don't stop it, a female Combee WILL evolve into a Vespiquen when it reaches a certain level. If you use a Thunder Stone on a Pikachu, it WILL evolve into a Raichu. These are things you can predict; things you know will happen.
Mutation is
random. Not every Carbink WILL mutate into Diancie, regardless of what you do to try and ensure that they would; otherwise there would be millions of Diancie running around the Pokemon world. That is the key difference between mutation and evolution. Of course in the games its a completely moot point, but that is what the Pokedex entry is implying, and why it doesn't just say that Diancie is an evolved Carbink.