I don't know... the Addams Family siding with the outsiders, the ones rejected by the mainstream of society, seems perfectly in character for them.
That's actually part of why, to me, it didn't work.
The Addamses are kind of parodies of old money bluebloods and part of their humor is that they don't see themselves as outsiders rejected by the mainstream, but as perfectly normal people. If anything, they see themselves as above the mainstream.
I'm not saying they see it that way consciously, I'm just saying that, from a metatextual perspective, it's a logical position to place them in.
Like I said, I don't for a second believe that Wednesday was trying to make a political or ethical statement about the repression of Native Americans, because, let's face it, that kid is hardcore evil. She'd probably consider the Trail of Tears to be a laugh riot, if she ever laughed. It was the camp counselors who chose to cast Wednesday and the other social outcasts as racist caricatures of Indians. The revolt Wednesday led used that iconography because it was what was already imposed on them, but the purpose behind it was to rebel against marginalization. As you say, the Addamses consider themselves not only normal, but entitled by their wealth and family traditions to do pretty much whatever they want, without giving a damn about external standards of normality or acceptability. The counselors tried to restrict Wednesday and the other "unpopular" kids within a cage of such standards, for which the Pilgrims-and-Indians iconography of the play was symbolic on the filmmakers' part, and Wednesday refused to submit to that. She wasn't standing up for anyone's rights but her own, Pugsley's, and Joel's. She was making a stand on behalf of the Addamses themselves and those they liked, period.