I've been on Tumblr enough to know that it's a very varied site in terms of what's on it, and while the prevailing political opinion is probably somewhere in the "SJW" sphere most of the site's content just isn't about politics.
Keep in mind, though, that while Reddit and 4chan do oversimplify Tumblr, Tumblr and its ultraprogressive amigos do the same thing to those sites. Hell, in just the past few months there have been multiple anti-Reddit and anti-4chan media blitzes based on the actions of reprehensible individuals. It's not like Tumblr get singled out for strawmanning and cherrypicking.
As for cultural appropriation, the exact cause of the butthurt depends on the culture being "appropriated". In the US, non-black appropriation of "black culture" is a pretty complex phenomenon that genuinely deserves to be discussed, but "appropriation" of other cultures is more suspect.
From experience both online and in person, most cases of the latter involve second-generation immigrants from affluent families in their teens and twenties who experience pretty much zero oppression in America (and in fact are frequently part of ethnic/racial groups that are statistically better off than white people in most respects). Their parents (usually actually from another country) often don't mind or are even happy when white Americans are interested in their customs, while the kids are horrified because "their" culture makes them feel special.