Another commenter (who’s contributed code to Lemmy) pointed to a link that provides the specification for that field: “A simple, human-readable, plain-text name for the object. HTML markup MUST NOT be included.”
So in this case, it’s more that the JSON looks a bit ambiguous: ‘mediaType’ is only referring to the format of the text in a post’s body, but - unlike me - you’d also need to be aware of the spec to know that it doesn’t apply to the title.
Lemmy mostly federates with Lemmy, but everything else out there (PeerTube, PixelFed, etc) has been developed to work well with Mastodon (sometimes to the Fediverse’s detriment), so your Mastodon account should be all you need.