What was "glowing with an evil mockery of beauty" in the garden in "Rappaccini's Daughter" by Nathaniel Hawthorne?
The line you mention from "Rappaccini's Daughter" by Nathaniel Hawthorne describes the plants and flowers in the garden which was created and is maintained by Doctor Rappaccini and his daughter, Beatrice.
The protagonist of the story, a young man named Giovanni, has rented a room which overlooks what appears to be a lush and beautiful garden; however, as time passes and he looks more closely, Giovanni can see that there is something grotesque (exaggerated and unpleasing) about the plants and flowers in this garden. This passage contains the phrase from your question:
The aspect of one and all of them dissatisfied him; their gorgeousness seemed fierce, passionate, and even unnatural. There was hardly an individual shrub which a wanderer, straying by himself through a forest, would not have been startled to find growing wild, as if an unearthly face had glared at him out of the thicket. Several also would have shocked a delicate instinct by an appearance of artificialness indicating that there had been such commixture, and, as it were, adultery, of various vegetable species, that the production was no longer of God's making, but the monstrous offspring of man's depraved fancy, glowing with only an evil mockery of beauty.
We learn, of course, that these flowers all contain a poison which makes them deadly to everyone who does not wear a mask (like Rappaccini) or who has not inured themselves to it through consistent contact (like Beatrice and eventually Giovanni).
Making this observation is one of the first steps to Giovanni's understanding about what the Rather evil doctor, Rappaccini, has done to his own daughter and is trying to do to him, as well.