In fairness, I'm exaggerating just a little (whodda thunk!).
Tumblr can be as much or as little of a community as you'd like. Like twitter or Instagram, depending on who you follow (and what sorts of content those people repost), you do exert a fair degree of control over what you see.
But it's also infamous as the primary breeding grounds of the sort of obsessive, creepily sexualized, high-passion-but-higher-anger fandoms that tend to earn entire communities bad raps. I'm perfectly able to carry on enjoying Doctor Who in spite of the people writing ten-thousand-word diatribes on exactly why they would or would not fuck Pete Capaldi's Doctor, but there's a lot of folks who find the whole concept irretrievably tainted by the association.