Is it useful to lessen the acne scars on the face using lemon juice?

No: It won't work. It's unfortunate that anyone has unwanted acne scars in the 21st century, when all acne can be managed quite easily, most cases by any licensed physician. A dermatologist can make the scars less apparent using lasers, fillers, abrasion, or surgery. If you want this, it's your right. Good luck.

Scars or Stains: All too often, patients refer to staining, or discoloration of the skin as scars. They are not. A scar is a dermal process (deeper in the skin) that resulting in depressions (depressed or boxcar scars) or deeper pits (pitted scars). Scars are not helped much, if at all, by any topical remedy. Stains, on the other hand, are.