God, does it really matter? I see so many complaining about the stereotype Tumblr has, but I've seen very, very few complain about, say, how 4chan is defined by /b/, or how Facebook is defined as bland and mainstream. Pretty much all the major websites have got some sort of shorthand to them. Maybe it's a flaw about the format that's blown out of proportion, maybe it's a subculture on the site that in no way encompasses everyone, but it ends up defining the site because that's the most unique thing it offers. 4chan offers trollish assholes, Twitter offers self-obsessed nuts, 9gag offers unoriginal content thieves, Tumlbr offers militant SJWs. They might not make up any significant majority, but they exist in that format in greater numbers than they do on any other. That's why those platforms become associated with them. But the only ones I commonly see complain about getting that treatment are the tumblrettes. Why? Why should Tumblr be exempt at all?
And it should be noted that I frequent Tumblr, and have for a little over a year now. I know that SJWs don't make up anywhere near the majority of the site - that dubious honor goes to crazy, obsessive "fans" and unoriginal bloggers who always seem to respond to things with "I CAN'T EVEN" or "OMG CRYING." And if it weren't for the SJWs, those subcultures are the ones that would be generalized as making up the entire site. And then everywhere would make fun of them because of it. Because that's what they do. People make fun of 4chan for being made up of assholes, people make fun of Reddit for being a bunch of dudebros, people make of Tumblr for having whatever people feel they've got the most of. It's the same sort of tribal mentality that fuel sports team feuds or college rivalries.
You can try to ask people to look past that, but they're just going to find some new stereotype to associate with Tumblr, and use that to make fun of it. Just as Tumlbr users would find a way to make fun of 4chan and Facebook and Reddit, because we've all got that little tribal mind in one way or another.