Prep the veggies: Thinly slice the cucumbers (you can use a mandoline for extra-thin slices). Halve the cherry tomatoes, and slice the red onion if using.
Mix the dressing: In a bowl, whisk together Greek yogurt, mayo (if using), lemon juice, Dijon mustard, minced garlic, salt, and pepper until smooth.
Assemble the salad: In a large bowl, combine the shrimp, cucumbers, cherry tomatoes, red onion, and dill.
Add dressing: Pour the creamy dressing over the salad and toss gently to coat everything evenly.
Chill (optional): Let the salad sit in the fridge for about 30 minutes for flavors to meld.
Serve cold: Garnish with extra dill or a lemon wedge if you like. Great on its own, in lettuce cups, or on top of greens!