According to https://www.theverge.com/2019/3/14/18266013/tumblr-porn-ban-...
they've had a 30% reduction in traffic.
If that means all the weirdos with genitalia in their profile pics are gone and stop messaging me randomly (which is who actually left as far as I can tell), then good!
The remaining MAU might actually be the users they wanted to keep.
But as someone who never used tumblr for porn, I've seen no evidence that tumblr is dying for anyone other than aficionados of porn.