I suppose it could be classified as cheating to have a custom character model with loud beeping noises for footsteps or something, but yeah it's stupid when you're not even using them.
Nah, that would be a kind of cheat in games like Half-Life, where you can force other people's models to be something else (a common variation is players using bright orange or pink player textures so enemies are easilly spotted), but this is not possible in UT, you can pick your own model, but not not what other people use, so if you want a player model that's bright pink and makes loud noises when walking, it will only hurt yourself, it wont give you any advantage, quite the opposite.
Besides, in UT, any model/skin you want to use has to be allowed by the server (IE he must have it installed on the server, and declared in his server packages) or it wont show up (you will become demoguy instead of the model you wanted), so even using a model with an invisible texture is not going to happen, not unless the server host was dumb enough to allow it.. but if he's that dumb, then anyone who joins can play as that invisible model, making the playing field even (that server would suck, but regardless, anyone who joins would be able to partake in its suckyness).
Player models are not a cheat in any UT game, unlike in Half-Life and Source games, where it actually can be abused.
This also leads me to belive this is a bug and not intended, EPIC went to all this trouble making sure models and skins can't be abused in this manner, and then suddenly decide that they are a cheat? that seems strange to me.. granted, they have made some pretty bad decisions as of late, even downright dumb ones, but surely, they havent compleately lost it (i hope)?
Yea, those are all not cheating. Achievements should be just disabled for that match if using mutators or cheat codes, and not overall, and custom models are legitimate!
Yup. ^this^
It's fair enough you can't unlock an achivement when using a cheatcode, or running an unknown mutator that does who knows what to the gameplay (a mutator could be written that gives human players a huge advantage over bots, for instance, making it much too easy to get certain achivements offline), but they should only be disabled for the match in which they are enabled, not because you have 3'rd party mutators and maps installed, that's useless!
Again, i choose to belive this must be a bug..