Even before tools like the Android App Inventor were created, I had a suspicion that the majority of apps in the market weren't anything to write home about. After doing some digging, I think I've narrowed down the number of "good" apps available in the Android Marketplace.
Android Pit data shows that only around 6,250 apps are at least 4 stars (this includes only those apps with over 50 downloads, and 50 ratings to cut out some of those apps that are voted up by the developer and his/her friends). I'm sure this isn't scientific, but it's certainly a good estimation.
That's about 7% of apps. As we start seeing more submitted from App Inventor, it's feasible that number will drop even lower.