It is unfortunate that the mainline Mastodon server doesn't support proper multi-domain hosting/multi-tenancy, but that is just implementation detail rather than protocol issue. There are alternative servers that support multiple domains[1] and multi-domain support has been discussed also for Mastodon server[2]

[1] for example https://jointakahe.org/

[2] https://github.com/mastodon/mastodon/issues/2668

tumblr?

A "lite" instance of mastodon, which just hosts on profile should be possible though, it's just that noone has created it yet.

Edit: I'm mistaken, because it's all just ActivityPub on the fediverse, of course there are projects making stuff lighter. I haven't found a dedicated single user project that's more than a demo, but stuff like Plemora seems light enough