I would like to ask if hydocortisone cream help lightens a dark neck?

I wouldn't expect it to. The depth of skin tone is controlled by the amount of pigment generated by cells deep in the skin. The surface application of HC creams would not penetrate deeply enough to affect those cells.

Agree with Dr. Ferguson. for information on Skin lightening, visit this site: https://www.nhs.uk/conditions/cosmetic-procedures/skin-lightening/ Wish you good health!