That would depend on what you think is a valid motive to create a character.
My best advice on this is not use your own personal restraints and beliefs but use that of the character you create, create somebody that would benefit if the Stormcloaks won.
Valid as in a legitimate reason to become disenchanted from the Empire. Talos worship for example was somthing the Empire fought pretty hard to keep, having sacrificed the Imperial city itself in the war, but in the end had been left with no choice but to accept their demands for its banning.
I think a more reasonable thing would be to become upset over Dovahkiin's near execution, or the Empire abandoning Hammerfell when the Aldmeri invaded it.
Why do people always feel they need to be talked into doing stuff in this game. Either do it or don't. It's not like the world is going to flip upside down because you went with the Stormcloaks. It's not like it makes a real difference either unfortunately.
Because doing somthing without motivation or desire just to see how events play out isn't the way a lot of people like to play. if for example they can find a way to connect to a cause, they suddenly become lot more interested in the events that are playing out.