I made up ULFI because I thought MIME has some problems.

> JSON may not look efficient

Efficiency is not the only issue; there is also the consideration of e.g. what data types you want to use. JSON does not have a proper integer type, does not have a proper binary data type (you must encode it as hex or base64 instead), and is limited about what character sets can be used.

(Also, like other text formats, escaping will be needed.)

> I feel like "JSON over HTTP" is a subtle improvement over "custom text formats over telnet"

I think it can be, depending on the specific use; sometimes it isn't, and will make it worse. (HTTP does have the advantage of having URLs and virtual hosting, although I think it adds too much complexity more than should be needed.) However, I still think that DER is generally better than JSON.

> HTTP isn't the worst bootstrap layer the internet could use to build new protocols and apps on top of.

I think it depends on the specific application. However, even then, I think there are better ways than using HTTP with the complexity that it involves, most of which should not be necessary (even though a few parts are helpful, such as virtual hosting).