Ah, usually the image quality is on par with the port's capability on the laptop. This can depend on the HDMI specification of the dock though; a lot of modern laptops use either HDMI 1.4 or 1.3. Older than that and it will still work, but then it might suffer overall image quality loss.
So the docking station should not hurt HDMI quality (or at least not by much, depending on said reasons), but it would be best if you used the port directly on the laptop.