Karna did humiliate Draupadi but she had humiliated him at her syayamvara when she said she doesnt want a suta putra to take part.Agreed. But the key question is did Karna who is otherwise well known for his righteousness, goodness, charity, humiliated Draupadi or not ? The point is if Karna really did it, then Krishna would never have gone to Karna saying he is the true Dharma Rajan, & asking him to lead the Pandavas in the war. My larger point is that the entire incident could be cooked up as victors always glorify themselves and show the losers in a bad light.
So tit for tat.