You might feel like some people don’t deserve for you to be kind to them. Correct them, but do it in a way that you would want to be corrected. If you can’t, then eliminate yourself from the situation (if you can).
For people that others would make fun of or joke about, BE ESPECIALLY KIND to them. If they have some that that makes them feel peculiar or different, you being nice (or mean) can have a profound impact on them. Sometimes, it just takes one person to be friendly to help someone change their life.
You will never regret being nice.