It depends a lot on what you're moderating. Being a ROM Hack mod, for example, is probably more demanding than being say, a mod of Pokemon Trivia. Then there's the fact that moderating a forum and moderating a chat room/battle server are different beasts altogether. But in general I wouldn't say that it's easy or hard.
If you want a better idea of what modding a Showdown server is like, it'd probably be better to ask one of the battle server staff like Omi or Zeffy.