Depends on your circles. Tumblr, like Twitter, is really defined by who you follow. If you follow 'professional', curated blogs where the author is promoting their own content, they will care about SEO and discoverability and they will tag their own posts with topical hashtags.
But most personal blogs don't care about that, they tag their posts with custom, made-up categories or use the tag field as additional meta-commentary. There's a much-reblogged Tumblr post chain [1] that talks about this; ironically the only way I was able to find the post again is because one of the commenters tagged it with topical tags.
So Tumblr is an amalgam of discoverable posts, long chains, and many posts that are completely untagged. If the blog's author has disabled the built-in integration Tumblr has with making search engines aware of posts, a lot of this content can only be found by going directly to the source through a friend-of-a-friend.
[1] (shortened due to NSFW words in url, in deference to those at work) http://bit.ly/2cuwzE0