I’m very curious to hear the Protestant view of Fatima, considering that it involves the Virgin Mary appearing before witnesses, many of whom were skeptics.
Probably a demon, because the prophecies didn't come true.
In short, when a prophecy comes from God, it will come true. If it doesn't come true, it wasn't from God.
If what a prophet proclaims in the name of the Lord does not take place or come true, that is a message the Lord has not spoken. That prophet has spoken presumptuously, so do not be alarmed.
When you're dealing with human prophets, this can be the result of demonic trickery, but it can also simply be the result of them making things up or misinterpreting their own desires or impulses as being from God. When you have a spirit making prophecies, though, those sort of human limitations wouldn't apply: they'd either be from God (an angel), or they wouldn't be (a demon).
The Lady of Fatima predicted that WW1 would end in 1917. It did not. From the Wikipedia article:
In De Marchi's account, he describes Manuel Formigao, the priest who interviewed the children during the apparitions, as alarmed by a discrepancy between a prophecy the children reported, and the current circumstances. According to the children, their apparition predicted that the First World War would end on 13 October 1917. "But listen Lúcia," De Marchi reports Formigao saying, "The war is still going on. The papers give news of battles after the 13th. How can you explain that if our Lady said the war would end that day?" Lúcia replied, "I don’t know; I only know that I heard her say that the war would end on that day ... I said exactly what our Lady had said." Jacinta, the youngest child, was interrogated separately and said the same: "She [Mary] said that we were to say the Rosary every day and that the war would end today." World War I ended a year later on Armistice Day, 11 November 1918.
From this, we can conclude that whatever spirit appeared to the children in the form of "the Virgin Mary" was not of God.
I understand that there is a Church of the Most Holy Trinity in Fatima, Portugal, and that on May 30th, 2010, Pope Benedict said that it would be wrong to consider the prophetic mission of the apparitions at Fatima complete. It continues to be relevant, he said, in that it continually invites men and women of good faith “to save the city of man.” 500,000 people were gathered for the feast of Our Lady of Fatima to hear the Pope that day. Link: https://www.catholicnewsagency.com/news/19653/fatimas-prophetic-mission-not-complete-pope-declares-on-solemnity
I am not aware of any official Protestant statements about the events at Fatima, where three children had an apparition on May 13 1917 But, then, I'm not aware of a million-and-one things stated officially in the massive ranks of what can loosely be called Protestantism! I'm certainly not an official spokesperson for the branch I'm in, Reformed Presbyterianism.
Nevertheless, in all my 40+ years in such circles I've never once heard any Protestant speak about the events at Fatima, nor read any protestant articles on it. Indeed, I expect very few Protestants would know anything about the Fatima events. However, I know enough about official Protestant views on Catholic elevation and adoration of Mary to offer this simple answer:
Clergy, like Nestorius, have pointed out that the Bible never says that God was born of the Virgin. It only speaks of the incarnation of the Logos, not of his birth. He said that although such fathers as Origen and Athanasius had used the term theotokos, it had not been incorporated into either the Nicene Creed of 321 or the Constantinopolitan Creed of 381. (Nor is it used either in the Anglican Articles or in the Westminster Confession.) Protestants never speak of Mary as "Mother of God" nor do they adore her as any kind of medeatrix in heaven, nor do they pray to her or give her any kind of adoration. They agree she is "blessed of women" and have great respect for her honoured role as detailed in the Bible, but view Catholic reverence for her to be a form of idolatry. Therefore, they would not give such credence to the events at Fatima as to agree that the Virgin Mary appeared. Whatever the children saw and heard, and whatever has resulted since, is not anything Protestants would believe in as being from God.
I could cite many biblical texts to support the Protestant view, but I do not suppose you are looking for that, therefore I will end my answer here.
I have to say, as a former conservative Lutheran (Wisconsin synod) raised and catechized, and a person taking part in the non-denominational Charismatic movement, as well as having experience of a Southern Baptist going to an SBC parochial school for 5 years, I have not ever seen this stuff taken seriously. It is largely seen, depicted, or interpreted as superstition and demonic deception.
And I should add that even the Eastern Orthodox Church is not that receptive to it (lest this sort of thing undermine their own dogma), even though they have their own beliefs in regards to Mary as the Theotokos, intercession of the saints etc. (If you read Eastern Orthodox articles on Catholic spirituality there is often a lot of suspicion of spiritual deception, aka "prelest").
I joined the Coptic Orthodox Church years ago, that has similar experiences like Our Lady of Zeitoun, where one might have some more sympathies, but I still expect some reticence due to dogmatic differences with Roman Catholicism. (It seems the Ecumenism on this issue is more like Catholics being interested in stuff going on in Egypt than the other way around).