How about a burger? Delicious, but healthy?
I would consider the expression "Delicious, but healthy?" to be casual style in today's standard English. And that, yes, your interpretation is what was probably intended.
LONG ANSWER: How that expression "Delicious, but healthy?" should be interpreted probably depends on how it was said. I can see one very likely possibility, and maybe another possibility:
And each version would be said differently (w.r.t. intonation) by the speaker.
I would guess that the speaker most likely meant for the #1 version. And so, the rest of my post will be on that.
I'm seeing your original version to be an ellipted version of:
- It is delicious, but is it healthy?
Which is a coordination of two main clauses (with the coordinator "but"). For purposes of analyzing the ellipsis, we can split the coordination up so that it is two separate sentences. And then, we can ellipt out the leading pronoun "it" and the auxiliary verb "is" because "personal pronouns and auxiliaries, can be omitted from the beginning of a main clause in casual style." (CGEL, page 1540)
And so, we can then get your original version by reconnecting them together again:
Here is a link to a post that discusses this kind of ellipsis and which also includes excerpts from CGEL: https://english.stackexchange.com/a/160937/57102
Note that CGEL is the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language (CGEL).