I like your characters and this design is fun, but the colors need to pop a bit more for something like this. The text color makes it almost illegible if one were to take a step back from the screen.
For the piece as a whole, I know the subdued colors are kind of your thing and are fitting for your series/style. But for something more "iconic" or meant for a shirt (like this), the color choices could stand to be a bit more vibrant. Just my two cents.