Two images from the ICC (Instrument Context Camera, a fixed camera under the InSight lander looking at the area of the deployable instruments will be placed) have had the lens cover removed, but the picture still seems to have dirt. Was this intended or expected? an earlier picture for comparison:
The camera is mounted just under the landers top surface, and obviously was exposed to a large amount of fine debris being blasted about upon landing. The cover I believe is a heat triggered spring loaded cap. Now as to why it failed to keep the dust out requires access to greater knowledge of the camera and cover design; such as was it pressure sealed, what the pressure was in the space between the cap and camera lens, if not a vacuum what gas was in between the cap and lens, etc.. I suspect if the pressure was less than that on mars then when the cap opened the dust might have been drawn towards the lens.
The basic design I have looked at suggests the dust from the cap might be flung back up onto the camera lens from behind as the cap flips open under the camera.
There might have been a static effect on the lens that attracted the particles as well.
And finally if the surface contains frozen ice particles that melted due to the thrusters then this may have condensed on the lens to some degree and when the cap opened the dust may have stuck to this moisture (although I doubt this would be the case days later in the low pressure of mars).