It's not about the genre of music-- that's just where the word originated from. It's used as a derogetory comment to people who look/act like the original listeners of emo-- people who think being 'depressed' is 'cool', people who whine constantly about minor things, etc etc. So, if you whine a lot or you have a 'pity me' attitude, you're going to get called 'emo'.