Make dressing: Whisk together mayonnaise, sugar, and vinegar in a small bowl. Chill in the refrigerator until needed.

Make salad: Cook bacon in a large skillet over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on a paper towel-lined plate, then crumble and set aside.

Combine broccoli, onion, sunflower seeds, and cranberries in a large serving bowl. Add crumbled bacon and drizzle with dressing; toss until well combined.