It's not okay. I always make jokes about them with my friends (a la Cartman from South Park) but obviously would never say anything to a redhead or discriminate them.
No, it's not OK to make fun of ginger haired people, I have ginger hair and believe me, it's not funny being called names just because you have a different hair colour than most people.
I don't care anymore as an adult but I started dyeing my hair when I was in middle school because of how much people made fun of me for being ginger. Getting picked on for sticking out doesn't feel nice.