Specifically relating to glitches, I think that cloning items and even Pokemon is alright totally not because I do it myself. If you think about it, people only want more Master Balls to help with catching Legendaries or shinies, or sometimes both! I wouldn't dare trying to soft-reset for a shiny without a Master Ball on hand, to the point where I glitched about 8 of them on Crystal VC just so I have them as a safety net for my soft-resetting hunts. Similar thing with Pokemon; I don't want to trade my super rare event, but I know that someone else could enjoy it just as much as I do, so why not satsify both people?
Glitched Pokemon, on the other hand, aren't that cool. For example, you could technically catch Darkrai/Shaymin/Arceus in the Sinnoh games, but they won't get through Poke Transporter to the newer games, and therefore I can't consider them as legit or even legal Pokemon. In this manner, I would consider it cheating as by catching and trading glitched Pokemon, you're setting up a scenario where their new owner may not be able to transfer them up to newer games as hack checks get more thorough and ban Pokemon that were caught in illegal circumstances.