Place the chicken breasts in the crockpot.
Sprinkle the taco seasoning evenly over the chicken.
Add the salsa and diced tomatoes on top of the chicken.
Stir in the chopped onion and bell pepper.
If using, add the corn to the mixture.
Cover the crockpot and set it to low for 6-8 hours.
When the chicken is cooked, shred it using two forks directly in the crockpot.
Mix the shredded chicken with the sauce in the crockpot.
Warm the tortillas in a separate skillet or microwave.
Serve the shredded chicken in tortillas and garnish with cilantro, lime, and onions.