ANSWER: There are a multiplicity of issues and problems with purported visions of the Virgin Mary. The most well known vision, certainly in relatively modern times, is the example you give. Fatima, located in Portugal, is the place where in 1917 three children reported to have seen six Marian appearances (on the 13th of six consecutive months). One of the three children said the apparition did not refer to itself by name but rather as "our lady of the rosary." These manifestations (endorsed by the Roman Catholic Church) revealed the following three "secrets."
The first of three visions revealed the existence of an ever-burning hell populated by demons and the souls of humans. These beings have flames shooting out of them as they scream and groan from the intense pain. It also apparently stated that people need to believe in the 'immaculate heart" (of Mary) in order to be saved from hell. It also declared that the Pope would consecrate Russia toward this 'heart' and that her conversion would come. The last secret revealed was that the Pope would be martyred, as well as lesser Catholic leaders and others.
One of the primary assumptions regarding manifestations of the Virgin Mary is that those who have died are not actually dead, but rather live on as immortal souls in either heaven or hell. The belief that the "soul" of a person continues a conscious existence right after death is NOT a doctrine found in the Bible. Notice what the wisest human who ever lived had to say on the subject.
5. For the living know that they shall die; but the dead do not know anything, nor do they have any more a reward; for their memory is forgotten. 6. Also their love, and their hatred, and their envy, is long ago perished . . . 10. Whatever your hand finds to do, do it with all your might; for there is no work, nor plan, nor knowledge, nor wisdom, in the grave where you go (Ecclesiastes 9:5 - 6, 10, HBFV).
Evil lying angels (demons) certainly can and DO visit humans on planet earth. The Bible does teach about people who see a FALSE visions (not inspired by God) then relay what they saw to others as if the message came from the Eternal (Jeremiah 14:14). A good case of someone communicating with an evil spirit is when King Saul got a purported visit from the prophet Samuel (a demon pretending to be the prophet) when the former went to the "witch" (medium) at Endor in 1Samuel 28:3 - 25.
One way to judge visions (and anyone who claims to be a prophet) is to determine whether or not what they say agrees with prior revelations from God and whether or not they always come true if they are predictions (see Deuteronomy 13:1 - 5; 18:18 - 22). If a spirit makes any statements that do not agree with sound Biblical teachings or makes false predictions, then they are not from God.
Either the visions seen at Fatima were fabricated or what the children saw was NOT literally Jesus' glorified mother for some of the following reasons.
Praying the rosary in order to enter heaven, or trusting in the 'immaculate heart,' contradicts what the Bible teaches is God's plan for saving mankind (see our article on 'The Plan of Salvation').
We are NOT to pray to Mary since only God should be worshipped (see Luke 4:8, which cites Deuteronomy 6:13). It is only through the name of Jesus Christ we are saved (John 14:6, Acts 4:10 - 12).
Purgatory and an ever-burning hell is a LIE that is NOT taught in the Bible (see our material on Hell and Purgatory)!
Mary is not alive but DEAD. She is unconscious and awaiting the resurrection like ALL the saints.
The prophecies regarding Russian consecration to the 'immaculate heart,' the martyrdom of the Pope and Catholic leaders, and so on did not happen. This means the vision told FALSE prophecies!
There is not a SINGLE Biblical example of dead human beings (not a demon pretending to be one) manifesting themselves in visions to those who are alive.
As a final note, while it is true Mary was a virgin when she gave birth to Jesus (Isaiah 7:14, Matthew 1:23), referring to her as if she stayed a virgin is not the truth. She did have SEX after giving birth to Christ and bore at least four other boys and at least two girls (see our article entitled 'Did Jesus have brothers and sisters?')!