There is textual support to argue that the Stormcloaks are not as racist as they are often claimed to be, but also there are a lot of prominent instances where the game says that they are racist. So, no, I do not think "Stormcloaks are racist" is simply headcanon. You can argue that Stormcloak racism is overstated by their enemies, but even in the most charitable reading it is still very real and very much in place.