Yes, this indicates a leaking seal that is less effective.
Water infiltration is not often a concern with well maintained sliding glass doors, but the danger can be minimized by installing an additional layer of weather stripping. Your door's seal can be strengthened with new weatherstripping. Adding a continuous strip of foam tape or rubber tubing to the bottom of the door makes the task simple.
Sliding doors are infamous for having the propensity to lose their tracks, collect grime, or even become unsealed with time. Combining these problems with a downpour will likely result in a flooded patiodoor track, minor leaks around the frame, and, in the worst case, water damage to your home.
Thus there is a concern when you notice that the glass of the sliding patio door is very foggy.
