入美國境是否不可以帶指甲鉗?

Carry-on is not suggested. They may ask you to throw away.
Check-in luggage is 100% OK.

http://www.airsafe.com/danger.htm

Most cutting instruments, including carpet knives and box cutters (and spare blades), any device with a folding or retractable blade, ice picks, straight razors, and metal scissors with pointed tips.