Future of the series
I'm just curious if this series is going to end soon or reinvent itself. I mean, you're doing all right with the limits inherent to the series, but eventually it's going to need to change, otherwise become stale and repetitive.
The mouth theme is gimmicky, and for a two minute cartoon, it seems to work, but eventually it will get tiresome, if it hasn't already. The characters never leave the mouth, don't seem to have goals or ambitions, and don't really do anything warranting a desire to follow them from week to week. Of course, this is necessary due to the gimmick, however with such limitations, the series can never develop dynamic characters, or plots, be they complex or simple. The best it can manage is a series of gags based on minor development or premises. (Or racial stereotypes)
Of course, I'm sure you know all this, and never intended the series to get really in-depth or complex. And I'm sure you'll be able to judge when the time is right to retire or change the series. For now though, you might as well keep it up as long as people are enjoying it.