"Emo" refers to a type of rock music called Emotional Hardcore or Emocore, which eventually lead into screamo. It in no way ties in with self harm, therefore if anyone calls you emo just look at them and say "thanks, I do enjoy that type of music!" Unless you dont, then just look at them like they're crazy
Aside from that, jokes about self harm, as with other disorders, are just distasteful and insensitive. They're also hurtful. It can make people who secretly self harm not want to ask for help. And it can actually make people self harm just to get laughs or attention. We all need to start thinking a little more before we speak. You never know what the person next to you is going through.
And if you find someone who insists on making jokes about such things sarcastically tell them "
Oh you're so witty and creative, I've never heard that before..." Emo jokes are getting really old and over-used so challenge them to come up with something new for a change. Most people don't like when you insult their jokes or flat out tell them they're lame. They usually move on to something else then.
And as always, remember that no matter how many people make jokes about it, there's plenty of us who understand and care so you're never alone and will always have someone to turn to when you need help.