It really doesn't matter how famous, infamous, or anonymous you are. "Just joking" is not a viable excuse for your bullying behavior.
Like her or not, Kendall Jenner's story is just one example of what many people experience, whether you're a celebrity or not. It only takes one lame-ass comment to trigger the Twitter trolls into mob mentality and group troll an account or hashtag.
Stephanie Seymour has modeled for many of the high end fashion magazines during the course of her career. Like all of us, her past has had its ups and downs, both professionally and personally. She recently called out Kendall Jenner and Gigi Hadid, inferring that they are not really models, even though they are being professionally paid to do that job. Her comment was to conclude that the term "supermodel" did not apply to the new girls and rather that they were the "B***hes of the moment!"
Jenner called her out and Seymour's apology somewhere along the way included the "just joking" scenario.
If you have to use the term "just joking" to justify a remark you'd like to take back, maybe just apologize and just leave it at that.