Tumblr was founded by David Karp in 2007, and was acquired by Automattic in 2019. (You can read Matt Mullenweg’s announcement from 2019 here). Just the fact that Automattic is behind Tumblr gives me some mental peace.
I’m not recommending one over the other. Mastodon is more like the Wild West right now. Tumblr is always evolving but the platform itself is stable. Explore both and see whats fits you best.