This is not ok, but be cautious ascribing malice to what could better be explained by nerves. People get nervous when they encounter a stutterer and don't understand what is going on, so they try to cut the tension by making jokes. It's about them, not you. The best response (for everyone) is to very calmly and kindly explain "I stutter. Please be patient." Almost everyone responds with a look of dismay and multiple apologies (smile and be gracious). You feel better, and they don't repeat this mistake.