That bumper support is for a pre 87. The 87-93 had the fibreglass bumper support like this...
To me it looks like the bumper was trimmed too low (behind the cobra insert) and left no support to keep straight. That combined with black paint in hot sun has softened the plastic in the middle section leading to the droop.
I would try a board of wood or two as a spacer under the bumper cover (sitting on the support) and let it get hot in the sun and see if it straightens out on its own. Other wise you are looking at a replacement bumper cover.