Is it safe to use silicone gel sheet on acne scar on the face?

Yes: As long as the acne is not active and there is no drainage. This may take 4 months to see a change wearing it a minimum of 12 hours a day.

Yes: Though it would be safe, i doubt that you would be able to keep the sheet in place long enough for it to be effective. For silicone gel sheeting to effectively flatten raised scars it should be used for, at least, 12-18 hours a day. There are more "convenient" ways to try and improve scars on the face.