Is it okay to use Cosmelan 2 even if it's already expired?

I would advise against using a product which has expired. The product contains hydroquinones which should definitely not be used after it expires.

It's not the best idea to use a cosmetic product after expiration dates. 1. The concentration and strength of the active ingredients is compromised, and 2. It could actually negatively affect your skin. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."