I'm going to laugh at my short friend
Why would you do that?
I just want to laugh at them, playfully
Your friend may be insecure about their height so laughing about it is not very kind. You don't want something you're insecure about to be made fun of, right? Try telling jokes that do not hurt anyone's feelings instead.
It's just height joke, who cares?
Your friend very well might, and then they're not likely to want to be your friend any more. Others who see you pick at something people are insecure about, which they have no control over, may also find it distasteful and want to distance themselves from you. Is it worth the risk?