Can't say I'm surprised. Tumblr has been so full of (non child) pornographic and outright weird material that it was due to attract this sort of folk as well. It really detracts from the platform for the rest of us, who just want to use it as a micro-blogging platform sitting between Twitter and Medium (where strangely, Tumblr has been horrible in finding its position).
Either Yahoo should clamp down and meanwhile give the platform a solid update -- otherwise they'll probably shut it down sometime in the future. I can image the cost of running it must be pretty high.