Doing something to give you an advantage over other competitors is usually considered cheating. Since not everyone has access to hacking mons, it can be considered cheating.
Personally I dont think its cheating as much as it's just lazy.
If you want a shiny mon with max ivs in all the desired stats, AND the proper iv spead for the optimal Hidden Power, then you pretty much have to hack it because the odds are just so low.
Hacked shiny Legendaries with maxed ivs are totally cheating because there is no natural way to obtain them like there is with breedable mons. But it's mostly GameFreak's fault because they never should have allowed Legendaries to be shiny to begin with.