I don’t know where you got this, but I read through the documents of Fatima last night and could not find where the Blessed Mother told Jucinta or Lucia that many souls go to hell because of sins of the flesh.
She did say the Blessed Mother said there are souls in hell and purgatory suffering because there is no one to offer prayers and penances for them.
This doesn’t make sense from what we learned from the other approved apparitions.
I have a problem with Fatima, especially where it was Lucia who told Francesco what the Blessed Mother said, because he could see her, but couldn’t hear her.
Also, things that Lucia wrote in her memoirs decades later, she never mentioned at the time of her interviews about the apparitions.
Fatima is still personal revelation and not necessary for Catholics to believe.
Jim