Peel jicama and cut into sticks about ¼-inch thick, cutting away any browned parts. Peel and core apple and cut into sticks about the same size as the jicama. Add both to a mixing bowl.
Whisk together lime juice until blended and slightly thickened. Drizzle half of the mixture over the jicama and apple. Use a pair of tongs to toss together to coat.
Sprinkle on Tajin and toss again to coat evenly. Taste and add more dressing and/or Tajin as desired. Add chopped cilantro, toss again, and serve.
Notes
You can make this salad in advance and chill to let flavors meld.