Is it permissible to delay wearing the hijab to cover facial disfigurements resulting from an accident, knowing that not covering them exposes the girl to societal criticism?

Covering a woman's hair is obligatory by scholarly consensus, and what you mentioned about facial disfigurements does not permit uncovering the hair. As for people's criticism, whoever seeks Allah's pleasure, even if it incurs people's displeasure, Allah will spare him their burden. There is no harm in treating disfigurements.