Can I let my 5-year-old use calculator? She is curious to know how it works the addition sums in a jiffy. Will it hamper her skills?

If you think she won't get used to it then maybe yes. At this age, kids find it very fascinating to use a calculator, as it seems almost like magic to them :) If she is really curious, maybe you can let her try it once or twice. But don't make it a habit, as it can definitely hamper her counting skills. Also, most schools don't allow the use of calculators, so if your kid gets used to it, she'll have trouble in class.