The feature is strictly opt-in, and it’s easy to revert back. However, it’s also being joined by some tweaks to how you find content. Tumblr is now hiding explicit blogs (that is, those with sexually graphic material) from web search results for anyone who’s logged out or under 18. Direct links will still work, but you won’t stumble across something saucy by accident.
It’s easy to get around both changes, whether by leaving safe mode off (it’s optional for over-18s) or by logging in. As such, this isn’t so much an attempt to clean Tumblr itself as another attempt at cleaning up its image. It clearly doesn’t want to be defined by porn, and offering a safe mode could ease the minds of people who might otherwise shy away.