Community is a niche comedy which suffers from terminally low ratings like most other NBC comedies. One, because NBC picks up stupid shows in a futile effort to replicate Friends, two, because they all cater to a certain humor crowd.
I am fairly certain Community never shows any notable fluctuation in its ratings when TBBT is new and in repeats this season. Decreases in its season-to-season ratings are accounted to an overall atrophy of NBC's ratings in steeper declines than the other 3 main broadcasters and relatively unsuccessful shows to begin with almost always shed viewers every season until they have their most loyal fanbase left.
In addition, there is likely no correlation between intelligence/cleverness and show viewership. In fact, the likes of Two and a Half Men have more viewers who make over $100,000 than NBC comedies. Thus, 'successful' people flock to CBS (as they have several other shows over $100k as well), and those making so much money are "probably" 'smarter' than someone working at McDonalds for a living.
While it is quite true CBS programs to the lowest common denominator, this in no way ties to intelligent viewership. Not to mention NBC usually loses to cable channels in the ratings races while CBS dukes it out with Fox for the top spot. Niche programming on broadcast channels is a failure if you want to not have mediocre ratings.
As for good TV shows that haven't been said, may I recommend newcomer Person of Interest? As this is its first season, and it recently took an upward swing in the ratings thus making it likely to be renewed, it's easy to get into, and has a rather compelling backstory with many tendrils of plots. The first few episodes are rather slow and unremarkable, however, the show introduces many more relevant plotlines after like episode 9 or so and stops with the "only way to resolve problem is gun violence" nonsense that made it a clone of lowest-common-down-syndrome NCIS: LA and its ridiculous plots. Hawaii Five-0, while equally gun-happy, at least has less moronic characterizations and nicer scenery, as well as somewhat cohesive and realistic storylines.
Now there's just action and sneaky methods of resolving things, as well as the ever-suspenseful questions of just what will happen to the character of the day. PoI's recent uptick in quality makes it a "check it out" show imo, it's a procedural format (for good reason, they're the most popular format in the world [House, CSI, etc]) with plenty of twists and serial storylines woven in, so it's a cut above the day-in-day-out of the CSIs and NCISes, which while successful, quality shows, are pretty by-the-books procedural.