I don't believe it should be just application/json because it's a specific format of json. There could be multiple json representations of the feed other than jsonfeed that the server supports and the client could define which ones they Accept.

So the server could support all of the following:

application/jsonfeed

application/rss+xml

application/atom+xml

Who knows, maybe RSS and ATOM could be represented in JSON and have the following mime types:

application/rss+json

application/atom+json

If it's just an API response, and it is your API for an application called Widget Factory, then you can, if you want, have your own format:

application/vnd.widgetfactory+json

Generally, defining such a mime type should have some specification describing it otherwise no client can reliably implement a compatible client. Jsonfeed have proposed that specification.