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

Minimally. Generally speaking, steroid injections and/or prp injections will yield better results.
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.
Unlikely. If it works the result will be modest. Don't use it too early either. Wait until the scar has healed completely.