Editing historical characters seems a little bit like cheating. I get the point that it can be modded, but that is outside of the game. I mean, you can also make yourself the Khan-Emperor of the Mongols and the Grand Caliph of Persia through modding...but that's not really the same game anymore!
It depends. If you give your historical ruler best possible stats and abilities, then sure it's cheating. But if you just change him to homosexual kinslayer who is excommunicated then I would say that it's not cheating.
Personally all I wanted was to play as Christian version of Alp Arslan of Seljuk Turks or Nestorian Khan of the Golden Horde, but ruler designer doesn't even give you option to make your own custom Seljuk or Mongol leaders, because it doesn't allow you to edit NPCs to make them playable. Editing the game files to make those characters playable isn't very difficult, but I was hoping a way to do it in-game.