You experienced really bad RNG in your case but yes, the overall drop rates for mods are pretty bad, especially considering that you actually have to find a particular slot with a particular stat if you actually want to remain competitive. That's pretty much why I've only been farming (done now) the battles and filling in the holes using the shop. The Challenges for mods is an absolutely terrible use of your energy unless you're done farming regular cantina and/or like spending thousands of dollars on this game.
