Can a girl get pregnant if sperm falls on the undergarments of the girl?

Absolutely not.

Men ejaculate 300 to 400 million sperm in 1 ejaculation.

Sperm can not live for more than a day outside of the male penis (testicles) and it does not live in the vaginal area any longer than 3 days. If a sperm fell onto the undergarments of a female you would not see it, nor would it live long enough for it to get someone pregnant.