You can view all NSFW content on the iOS app, but you have to log into an account and click a button to say you're old enough. I think they're trying to make it effectively opt-in.
Another guess is that this is easy for Reddit to do (and not Tumblr) because Reddit has the concept of an NSFW subreddit, where all content under that sub is age-restricted. And content on other subs would be heavily moderated most of the time. Whereas on Tumblr, posts don't go directly to "pre-categorized feeds" in that same way, since you're just an individual making a post on your own page more or less. So moderation (I'd guess) would be a lot harder.
Another guess is that this is easy for Reddit to do (and not Tumblr) because Reddit has the concept of an NSFW subreddit, where all content under that sub is age-restricted. And content on other subs would be heavily moderated most of the time. Whereas on Tumblr, posts don't go directly to "pre-categorized feeds" in that same way, since you're just an individual making a post on your own page more or less. So moderation (I'd guess) would be a lot harder.