I can see what you are saying if you are referring to the Talk channels/streams. Other than Stern and Maxim, the talk channels get less bandwidth than the Music channels, so they are going to sound somewhere near AM quality.
The sound quality on the Music channels is very subjective. I have been a subscriber for over 2 years, and the sound quality has definitely improved. It takes a little while to get used to the compressed sound, but I do not think it has a mono sound. When I listen in my car on Polk components, the sound seems fine. I have an ancient Audiovox PNP1 receiver running through an AUX input on my Blaupunkt headunit.
What Sirius receiver are you using? Is is a hardwired to your headunit, or are you using the FM transmitter?