The quote you have cited occurs in Canto III of this great mock epic, and comes during the card game that Belinda plays with two of her suitors. Reference is made to various sylphs and spirits who sit on cards, protecting them for Belinda. Thus it is that the description of this game is then interrupted by the following small stanza, which incorporates the section of the text you have outlined:
Oh thoughtless Mortals! ever blind to Fate,
Too soon dejected, and too soon elate!
Sudden these Honours shall be snatch'd away,
And curs'd for ever this Victorious Day.
This quote of course foreshadows the "snatching away" of such "Honours" as winning a card game and also makes us expect some kind of traumatic, cataclysmic disaster. Of course, Poe uses this to poke gentle fun at the importance with which the lock of hair that was "raped" (itself an incredibly strong word to describe such an event) was taken. The stanza introduces a portentous tone through reference to "Fate" and the fickle nature of "Mortals," which are "blind" to the powers of destiny and can range from dejection to elation. It is important to note the mocking tone in this quote.
These lines use hyperbole (exaggeration) to mockingly foreshadow the "crisis" that will soon erupt: Belinda's loss of a lock of hair to the Baron. Horrors! The poem pokes fun at a real-life furor that blew up in the wealthy and prominent Fermor family after a man named Lord Petre cut a lock of hair from Arabella Fermor without her permission. The family completely overreacted. Pope in these lines mocks the way these trivialities, such as who won a card game or who took a lock of hair, can be elevated to the level of importance of the real subjects and themes of epic poems: war, death, destruction, survival and finding meaning in the universe.
In real epics (as opposed to the "mock" epic genre of this poem), such as the Iliad, fate is important and the gods actively intervene in the lives of humans. Pope parodies this when he writes of these completely trivial events: "Oh thoughtless Mortals! ever blind to fate!" If powerful people are blind to fate in a real epic, this can cause thousands of ordinary people to die or suffer. Here, these events cause no real suffering. Pope thus uses hyperbole in these lines to puncture upper-class overreaction to very minor events. Further, by calling winning a card game "Honors" or the time of winning it "this Victorious Day," Pope compares it to the honors and victories of an epic poem, which would be, for example, to win an important military victory. By applying language normally reserved for important events in the life of a society to something as silly as a card game, Pope highlights and makes fun of the way upper-class people lived in his time period.