"Fanciful" is an adjective describing something that has been made up and represents an unreal world of imagination. So, you are asking how successful Spenser is in creating made-up, unreal worlds of imagination. His own words answer this best:
But to the pray when as he drew more ny,
His bloody rage asswaged with remorse,
And with the sight amazd, forgat his furious forse.
In stead thereof he kist her wearie feet,
And lickt her lilly hands with fawning tong,
As he her wronged innocence did weet.
O how can beautie maister the most strong,
And simple truth subdue avenging wrong?
In Spenser's world, a hungry fierce and "ramping Lyon" is charging toward the resting Unaes, forsaken by her Knight, but stops, takes pity upon her and, instead of devouring her alive, licks her feet and hands. And we believe this world, this "lyon" and this happenstance: our emotions respond according to a true account. Spenser is extraordinarily successful in "creating fanciful worlds in the Faerie Queene."