[In advance, I will say that this is a bit tongue in cheek, but I think the facts and arguments are valid.]
I think you are expecting answers to indicate that it is one or the other, but I think the answer to the literal question is "No" - it is neither intended as a miracle, nor is it a testimony to the superiority of some divine diet.
First, the Hebrew term translated by the NET version as "the king's rich food" literally means "the king's rations" or the "king's portion". The JPS Tanakh translates the phrase "of the king's food". The Septuagint reads "the royal provisions" (τὸ βασιλικὸν δεῖπνον). Even though there is nothing in the proto-texts to imply that the king's food was somehow more nutritious than what Daniel, Hananiah, Mishael and Azariah ate, some translators assume that it must somehow be the case.
Second, the Hebrew word translated by the NET and other modern versions as "vegetable" represents seeds that are used for food - i.e. legumes - specifically. This is reflected in the choice of the King James translators in using the word "pulse" - an archaic word for "legumes". In the Greek Septuagint, the underlying Hebrew was translated as ὄσπριον - also meaning "legumes".
The foregoing considered, the following translation is more accurate:
At the end of ten days
their appearance was better and their faces were well-nourished they looked better and
healthier compared to the young men who ate the king's
So the guard took away
their rich the king's food and wine, giving them vegetables legumes.
The Concise Oxford Dictionary provides two definitions for the word "miracle":
An extraordinary and welcome event that is not explicable by natural or scientific laws, attributed to a divine agency.
An amazing product or achievement, or an outstanding example of something
The event being judged is the
Daniel's, Hananiah's, Mishael's and Azariah's being better and healthier - after eating only legumes and drinking only water for ten days - than those who ate of the king's provisions and drank wine.
Taking the first definition of "miracle" to be the relevant, to argue that this event was a miracle would require proving that there is no way that it could be explained by natural or scientific laws.
But that clearly is not the case here. It is possible, for example, that those who ate of the king's food ate too much and appeared bloated or drank too much wine and had bloodshot eyes. In either case, they would not have appeared "better" or "healthier" than the Hebrews youth's.
Testimony to the superiority of the divine diet?
The argument here is that a diet of water and legumes ("the divine diet") is superior to a diet based on the king's provisions and wine.
Again, Scripture does not tell us how much of the king's food they ate or how much wine they drank. If, in fact, the other youth's were immoderate, it is completely possible that the Hebrew youths would have appeared even better and healthier if they followed the same diet in moderation. Similarly, it is also possible that they would not have appeared better or healthier than the other youth's had they eaten and/or drank more and/or less.
Furthermore, even if we accounted for quantity and assumed that each set of youths ate and drank in moderation, there is no way to guarantee that a diet that seemed to be more effective for four particular Hebrew youths would be equally effective for all of mankind.