The question is whether or not Tumblr users would want such a thing.

I feel like the same thing will happen like when WordPress introduced a (bad) TikTok/streaming clone called Tumblr Live. I think less than 10% of the userbase ever interacted with it, most of the community openly hated it, and the people who did use it largely didn't use Tumblr themselves.

I could see Tumblr users actively finding a way to defederate their blogs from everything Fediverse related.