The fox was known as a clever animal in ancient times just as today. Thus, Jesus may have simply been calling Herod Antipas crafty. However, the fox is also an "unclean" animal in Judaism, so the word was not merely a positive reference to Antipas' intelligence.
Looking behind the Greek text, where the word is clearly "fox," is a Hebrew or Aramaic word that Jesus must have actually used. The Hebrew word שׁוּעָל (shu'al) can be translated either as "fox" or "jackal." If Jesus meant "jackal" then the word would definitely be an insult. Moreover, Jewish tradition did not think kindly of foxes either. Talmudic references to foxes include "Be rather the tail among lions than the head of foxes" and "The lion has become a fox."
Conclusion: What exactly Jesus meant by the term is hard to know. At best, he was calling Antipas crafty. If Jesus was thinking of the "lion vs. fox" theme, then he could be comparing Antipas (a mere fox) to his more formidable father, Herod the Great. If Jesus actually said "jackal" in Hebrew, then the insult would be stronger, like calling him a wild dog or a devourer of dung and human flesh.