You get this trophy in the last desert section before the sunken city slide (after the bridge where you get the Threshold trophy). You can use the first door on the left in the chapter select hub to reach it.
After releasing the first carpet creature, you follow it to a second structure at the base of a large dune and release some more. Climbing to the top of the dune, you can see the mountain ahead and a structure below. Rather than going there, turn right and head along the dune.
To your right as you proceed, you'll see a hollow where 3 dunes met. Visible in the centre is a flower. Move down to the flower and make sure you touch it.
The Mirage Trophy will pop after a few seconds
Video for this below: