Of course it's mostly Nords. It makes more sense for them to recruit the local Nords rather than march an entire legion of Imperials into Skyrim. The Nords are used to Skyrim's cold and are more familiar with the terrain. As Rick said, it's a civil war. The Nords are basically fighting each other. One group is with the Imperial Legion, the official army of the state (ie, the Empire), while the other is for the Stormcloaks (ie, the rebelling army). While I, too, would like to have some diversity, it does make sense that the bulk of the armies are mostly Nords. Just like in real world civil wars, the bulk of the population in both armies are from the nation currently fighting itself. There are, of course, foreigners who would come assist one side or the next, but the bulk of the soldiers are going to be native to that particular country.

As for the Thalmor, this is exactly what they want. A civil war means less resources for both Skyrim and Cyrodiil when it comes time for the Second Thalmor War.