Unfortunately stuttering is pretty easy to make fun of. It’s rare to run into another person who stutters. Heck, even if you did you may not even know it. Many of us get very good at hiding it and unless someone knows us well they won’t associate stuttering with a brief block or stammer that happens in conversation. I’ve learned not to take jokes like that too seriously even if they do make me a bit sad inside. Fluent people really have no idea what stuttering is like and if they did they likely wouldn’t joke about it so openly. Also, it’s good to laugh at yourself sometimes.