Sunscreen blocks UV, not cosmic rays, which is the radiation for which jetliner passengers receive an unusually high dose. The glass in the windows probably blocks 75% of UV, and UV exposure is associated with other types of skin cancer, not melanoma (except in cases where one has a history of bad sunburns). In fact, in this review, melanoma is said to be inversely correlated with occupational exposure to UV:

https://pubmed.ncbi.nlm.nih.gov/9335442/