Can hydroquinone cream help to lighten a scar due to stitches?

Unlikely: If it works the result will be modest. Don't use it too early either. Wait until the scar has healed completely.

Scarred scar : It may, although its generally better for hyper-pigmented lesions. A corticosteroid cream may work better. The location of the scar is important.

Yes : It will take time. You may benefit from laser resurfacing.

Minimally: Generally speaking, steroid injections and/or prp injections will yield better results.